Keep in mind that part of the beauty of Guild Wars is finding play styles that
suit you.  There is no uber class combination or attribute distribution that
you need to go into in order to be worth something as a player.  To do that,
you need practice, knowledge, cooperation with your group and finally a little
patience with those who don’t have the practice or knowledge yet. (Looking down
on noobs isn’t a good philosophy, but looking down on people who won’t listen
to the rest of the group is a healthy attitude if you ever want want to make it
through the game.)  At any rate, here are some tips and skills that you can try
for yourself.  Decide what skills you’re best with and what skills work well
together, and always be ready to experiment.

For PvE- Quests, missions, and everything in between:

Make sure you’ve got some points in soul reaping, because all the weak
creatures are going to keep you supplied with energy as they continue to die.
The greatest dangers in PvE are facing too many opponents at once, and while
experienced rangers in your party will try to prevent that from happening by
luring a few enemies towards your group at a time, in the case of a battle
against too many enemies, your teammates might find their reserves running dry
while you continue to have plenty of energy.  In worst case scenarios like
that, you’ll be glad you have it.

Death necromancers will find that the large numbers of corpses are even more
important for making their minions with.  For much of the game, you’ll only
have bone horrors to choose from, but when you get the spells animate bone
fiend and animate bone minions, you’ve got to decide which suits you best. 
Horrors only require 15 energy to animate compared to the 25 needed for minions
or fiends, so if you’re more concerned with using the energy on other skills,
you may want to go with horrors or not raise the undead at all.  If you want to
focus on having your army be powerful and long-lasting, I’d recommend fiends,
which seem to be more powerful than the other types and attack at range. 
Minions are best when you want numbers.  Since you get two from each corpse,
you’ve got double the minions for your enemies to dispose of, and double the
energy from soul reaping when they die.  Minions don’t last long, but that’s
easily turned to your advantage by casting death nova on them and using them as
kamikazes and batteries all at once. I’m honestly not that sure what other
death skills are best for pve, but well of suffering or putrid explosion would
be a good alternative to raising minions, and deathly chill and deathly swarm
offer decent damage to enemies.  For pve death necros, I’d say making hordes
and hordes of the undead is going to be your best bet.

Blood necromancers are going to be relieved that their attacks keep them
healed, because they might find themselves soloing quite a lot thanks to how
underrated they can be by pick up groups and mission parties.  From early on,
blood necros are well equipped to be a fairly self-sufficient halfway-decent
damage dealer, and this is what I’d recommend in order to be one:
  -First, shadow strike.  The spell takes two seconds to cast, which is a
little dangerous, but as long as you cast it when the enemy is above 50%
health, you do a lot of damage, and you’re healed half of it.  It’s not a bad
idea in pve battles to heal yourself by turning from a wounded opponent to cast
shadow strike on one with decent health.  The sad thing about shadow strike is
that it needs to be used early so you get the full damage, but you probably
don’t need the healing at that point. Oh well. 
-Second, life siphon.  This is not a great attack skill and you’ll rarely do
any severe damage with it, but it is worth casting towards the start of a
fight, because it lasts a long time and drains a fair bit of health over that
time, and it’ll keep you steadily regenerating your own health.  The recharge
time is very reasonable too, and so as long as you’ve got the energy to keep
doing it, you can give yourself a very nice amount of hp regen, and as I’ve
said, it does last for a while.
-Third, barbed signet.  If you have a lot of health (For instance, if you’re a
primary warrior)   you don’t want to use this, because you’ll be hurting
yourself more than your enemy, and you’ll be wasting a skill slot to do it. 
If you’re a primary necro with a lot of points in the blood attribute, here’s
how it works.  For zero energy, you do a fair bit of damage to your target and
heal yourself a bit.  It’ll take thirty seconds before you can do it again, but
it’s a good skill to have when you’re finishing off an enemy and shadow strike
can’t heal you and only does half its damage.
-Fourth, dark pact.  This one pisses off your own monks to no end, because
you’re doing damage to yourself while you’re damaging the enemy and because of
the low energy cost, casting time, and recharge time, it’s a rapid fire attack
for when you’ve used everything else and need to keep doing damage to end a
fight. As a result, it looks like someone’s doing a bad job of trying to kill
you, and your monk may waste time and energy healing that back. The ironic
thing is that the necro’s sacrifice skills balance very well with the healing
skills, so the monk has no reason to bother. Once you’ve spammed dark pact a
little, turn to the next target and use one of these other three to get that
health right back and start again.  (I should also add that some players like
using dark pact in combination with the death skill dark aura to do a lot of
aoe damage around themselves very quickly.) The best sequence for this blood
chain is life siphon, (not necessarily on the person you’re attacking if you
want to make the most out of it, but using it on your target will certainly
help kill it.) shadow strike, dark pact, barbed signet, dark pact, dark pact,
dark pact…. Watch your health when you start using a lot of dark pacts in a
row, however. Other things a blood necro might want is vampiric gaze for
healing oneself with a ranged attack, vampiric touch for when an enemy warrior
gets too close, and soul feast, touch of agony, signet of agony, plague touch,
and that aforementioned dark aura for when you want to rush in like a lunatic
and start doing a lot of damage at melee distance.  This is probably a bad idea
in most cases.  Well of blood is a plus though.

For pve, I don’t really expect many people to go into curse necromancy, but if
you do, adapt yourself to the biggest threats of the region.  The most common
monsters do physical attacks, so faintheartedness, insidious parasite, and
price of failure are all going to be pretty useful. Plague sending might be a
nice thing to have considering how many monsters love to give out conditions. 
Suffering is nice, soul barbs might be a good way to do some actual damage with
your hexes, and so on.  Tell me if you work something really effective out.

In pvp, the roles are reversed.  Blood becomes less important, and curses
become the king of necro attributes.  Don’t worry though, you can make the
switch when you get there, and you can easily spread your points throughout
several attributes.  I love refund points. As an example, my current setup is
11 Blood, 10 Soul Reaping, 10 Curses, but with runes and facial scars, It’s 14
blood 12 soul reaping 10 curses.  In pvp, you can try using that pve setup
you’re so used to, and it’s good to start there and adjust over time,
especially if making the jump to curses, but you’re going to have to learn to
change your attributes and your skills constantly to suit the build your group
wants you to have.  Here are some of the pvp roles necros play and some of the
skills people want you to have and to bring.  I didn’t talk about elites in the
pve section because most players wont have those skills for much of the game,
but if you’re going to the tombs, you need to start getting all of your elite
skills.

Death necromancy- The minion master builds are pretty popular in tombs lately,
so death necros will have some opportunity to ply their trades.  As I’m not a
death necro, I can’t give detailed advice about the builds they run, but they
usually use minions, and they often have another person such as a blood necro
with blood is power or a similar health-sacrificing spell use it repeatedly in
order to quickly kill themselves before the match starts.  The corpse is used
to make a minion and resurrected right afterwards, and they try to do this as
many times as they can in order to raise a huge army by themselves.  Blood is
power is especially useful for the sacrificer because he’ll be giving the
minion master energy regeneration to make all of the minions with.  The result
is that the blood necro starts out with a lot of death penalty, but the minion
master has plenty of minions when the round begins.  What he does with them,
I’m not quite sure.  Attacking people, batteries, etc?  They’re a nuisance and
they help him. The other death skills that people want from necros are putrid
explosion and the elite tainted flesh.  Putrid exploits corpses to make
explosions that do a very nice area of effect damage to all enemies around the
corpse, and it’s a good way of getting rid of bodies before the other team’s
necros can use them.  Tainted flesh is used on an ally that’s being attacked by
melee weapons (look for the other team’s warriors and cast it on their targets,
naturally) and everyone who attacks them in melee is diseased.  The disease can
spread, and if people are running this build, they’re going to want to use the
mesmer skill epidemic to spread it among everyone in the other team.  Disease
isn’t that bad, but having your entire team diseased is a nightmare for enemy
monks and so tainted flesh makes it much easier to kill the other team during
the resulting chaos.

Blood necromancy- Well of power might be worth using if your team tends to
stand close together and has a lot of casters, but I’ve rarely if ever seen a
tombs group use it.  Soul leech is a great anti-caster elite that acts like a
version of the mesmer skill backfire that heals you.  A particularly vicious
N/Me might find it fun to cast both soul leech and backfire on a caster and
dare them to keep using spells.  Be wary about blood hexes in pvp however, most
good teams have monks and most good monks get rid of hexes. For this reason,
life transfer is pretty much useless in tombs, although it’s pretty nice in
competition arenas.  The penultimate role of a blood necromancer in tombs is to
use blood is power to give your team’s monks energy regen so that you never
have to hear your monks speak those famous last words: “I’m out of energy” that
so often marks the point when your team is massacred.  One team build that has
been given up before I ever got to tombs is using order of the vampire again
and again to let your teams physical attackers share in that self-healing
vampiric damage like you enjoy.  It sounds cool, but it must not have been good
enough to merit building a team around and spending an elite on.

Curse necromancy- Here you have two major focuses to decide between.  Does your
team want you to help by making sure the group’s target is defenseless or by
making sure the monk can’t help him for long?  If you’re helping the team kill
someone, your staple skills are lingering curse and rend enchantments.  If your
team uses a lot of warriors or archers, make sure to use barbs, weaken armor,
and rigor mortis.  If your team uses a lot of necros and mesmers, soul barbs is
vital.  I don’t think the effects of defile flesh and lingering curse that
limit the effectiveness of healing stack, and if not, don’t bother to bring it.
(If they do stack though, your target only receives one third of healing, so I
need to test this sometime.) Experiment with the right skills to bring based on
your team and your expected target. If you want to stop a monk from healing or
protecting, you’re not as well qualified as a mesmer would be, but by casting
wither and malaise you’ve given your target negative four energy regeneration,
something particularly awful for a monk. Spinal shivers lets you continue to
interrupt your target again and again and again without having to wait for
another spell to cast, as long as you have a weapon that will let you cause
cold damage. (Some curse and death wands/staves do, so no troubles there.)
Bring some mesmer skills if this is your aim.