How do you say i have a boyfriend in french?

I think you made a typo. If you want to say, "Will you be MY boyfriend?" you could say this:

Veux-tu être mon petit ami ?

sounds like: vuh-TEW EH-truh moh[n] puh-TEE-tah-MEE

The [n] indicates that the "oh" should be nasalized. The Word "mon" should sound like the "mo" in the English Word "mole".