What should I do in my first ?

I am 15, i have had like.. a lot for my age. At least 20 times by now, I'm pretty comfortable with it by now. This has actually helped my popularity a lot, I live in utah and I am considered the best looking male skank in school lol. This girl wants to have a three some with me and my best friend, she thinks we are both hot and wants to try it. Me and him and both pretty comfortable with each other but I just don't know what boundaries we have. How should I go about doing this?