For the Necromancer himself you can use Regrowth Belt, Regrowth, Drain Life, Drain Soul, and Death Link. I wouldn't advise using direct healing spells from the holy school like Minor Heal. You should also stay away from using Vampirism because of the Necromancer's frailty and lack of strong melee options compared to other mages. Also consider using defensive cards that prevent incoming damage rather than needing to heal as much later, such as Block, Brace Yourself, and persistent sources of armor.
If you have skeletons, you can reconstruct them with Reassemble, Unholy Resurgence, Mort, and Acolyte of the Bog Queen.