Basicly there are many diffrent ways of playing necro. Maybe one of the classes that can have many diffrent specs for diffrent bossfights and so on but i'll try to keep it as simple as possible for you.
Reanimation is the pet tree.
Nightfall is buffing the necros damage.
General Survival and -hate.
When i level a Necro i try to get the first 10 points in reanimation. first 5 and 5 points in infenstation. After that i go NF (nightfall) and go all the way there before going back on reanimation and when you hit 80 it should look something like this.
http://goo.gl/nSWxaT
Simple way of playing necro.
Flesh to worms, Pestilential blas and mark of devoer should always be on the mob. (you will only have worms and pb in the start)
Ice strike - main damage spell after dots
Blasphemus ruin - area damage around you (hits like ice strikes but takes half the time to cast)
life strike - heals you a bit
Me personally i like to spec Despoil the soul x8. Which removes all pets but gives you more magic damage. It's good cus then you have to learn to rotate your ccs and learn what spells to use cus you can't rely on any pets. This is also really good for leveling if you have some skill, if not it sucks
gl