Would it be bad if my friend and I messed around $exually?

I'm a 15 year old guy. And I have a really really good girl friend that is 14. We have been talking lately about doing some fun stuff. I have a treehouse that's really nice. And would it be bad to invite her over to swim and then we go into the treehouse and mess around? Would it be bad if we made out naked? I don't want us to be different in a bad way. Would that be okay? What about nervous or something? I want to do stuff with her and she said she did too. But everytime we talk (text) about it she doesn't know what she would do and such. Please tell me what I should do