Formal:
* Vous avez mon cœur. (This is the most literal translation and is very formal)
Informal:
* Tu as mon cœur. (This is the informal version, used with someone you know well)
* J'ai le cœur à la main pour toi. (This means "I have my heart in my hand for you" and is a more poetic way of saying it)
* Tu m'as volé le cœur. (This means "You stole my heart" and is a more romantic way of saying it)
The best way to choose the right translation is to consider who you are talking to and the situation. If you are talking to someone you don't know very well, it's best to use a formal translation. If you are talking to someone you know well, you can use a more informal translation.