I think my cousin was my first kiss
Did you ask your cousin if he was your first kiss?
I am scared to ask him because I might be wrong
There's nothing wrong if you two were young or it was just a friendly kiss.  That being said, if you were older or it was a serious kiss you shouldn't engage in that kind of behavior with relatives.
I'm really scared to ask him
I understand.  It's difficult to deal with delicate situations like this.  Perhaps it's best to pretend nothing happened and forget about it, if it was just a one-time thing?