Once your Necromancer reaches around level 75 (perhaps with a help of a leveling build), you will be in a prime position to pick and work toward your endgame Build. Keep in mind that your Build will not only define your skills going forward, but also your optimal weapons and armor.
Below you can find some of the best Necromancer builds for Diablo 2.
Poison Nova Necromancer
A Necromancer build focused on poison skills, especially Poison Nova, as well as using Curses, Summoning Spells, and Poison Resist negation gear to make poison more effective. Made much more powerful with Update 2.4.