"Comment vous appelez-vous ?" (formal)
"Comment t'appelles-tu ?" (informal)
Explanation:
* "Comment" means "how"
* "vous" is the formal "you"
* "t'appelles-tu" is the informal "you call yourself"
* "appelez-vous" is the formal "do you call yourself"
You would use the formal version with someone you don't know well or someone older than you. You would use the informal version with friends, family, or people you know well.