Hey, you should try some of the builds on this site: http://metabattle.com/wiki/MetaBattle_Wiki
The two main build are Necro Terrormancer and Power necro (you can find it in the “Ranked Arena”)
It helped me a lot when i started necro. Since then i adapted the builds with my way of playing necro, but they are still very close to the build i found on this website.
The Terrormancer is a condi build with multiple fears to be able to apply your condition without dying too fast, Power necro god an insane dps around his auto-attacks (on death shroud, liche, and even normal form hurts)
If you have any question about builds and other stuff, mp me