Based on the skillsets used. If you're not sure, just choose what you focus MORE on.
If you "don't fit" in this list then choose what is closest.
This is based on your favorite way to play not what any specific character currently is.
EDIT: A note on classes.... there are no classes in this game. Only 3 skill categories based on 3 generic classes.
Here are the categories and skills)
Warrior: smithing, heavy armor, block, two-handed, one-handed, archery
Thief: light armor, sneak, lockpicking, pickpocket, speech, alchemy
Mage: illusion, conjuration, destruction, restoration, alteration, enchanting