Question: Skyrim how do i become a neromage?
Make conjuration one of your main skills and focus on the part of the conjuration tree that has to do with necromancy. That's what I've done with one of my characters.
Answer:
Technically necromages don't exist. Necromancer is the word you are looking for. The other answers covered the rest :)
I'm going to assume you meant "necromage". Necromages specialize in death magic. Your best bet would be to max out Destruction and Conjuration along with a good amount of perks in Alteration and Restoration. Illusion would be the school you wouldn't be using.
You can't really become a necromage...
you can, however, buy frost spells from wizards at college of winterhold