This is a wonderful question, @anne2! I’ve opened it up into its own topic so it can get the attention it deserves 
I personally do not know much about either Necromancy or Demonology/summoning demons. But you may find these resources to be helpful:
- Demons: Demonology, Magick, and Psychology
- The Greek goddess Hecate is associated with Necromancy and could be used in spell or ritualwork of that nature
- Black Tourmaline, Obsidian, and Tiger’s Eye are crystals worn for protection and can prevent possessions.
I also agree with @Amethyst’s adivce- cleanse and seek out protection spells! A strong defense is often the best offense
Wishing you the best of luck, Margaret- blessed be!