Am I gay?

Well, I think that maybe when you make those comments, it might be that subconsciously you are trying to see how they react, being flirty with ones language is a sign that there is an attraction there and maybe you are looking for a positive response to see that one of your friends finds you attractive. You are young. Right now you are going through physical and mental changes that can put anyone in a mass state of confusion, sexual or otherwise. I have had many gf's that have had sexual feelings for women even though they love having sex with me, a man. So I think you are ok. On the "what should I do" part. If a friend gets upset when you say she has a nice butt, she is probably not into it. If she responds and jokes with you sexually, there might be a chance at kissing or whatever you want.

i have the same problem especially when I see a girl that has big breast and a lot of ass.

You're young and still trying to figure out how you feel about people-- soon you won't be feeling so confused and you'll understand you're sexuality!

It's perfectly normal to have these feelings at your age, and there's nothing you can do to stop them, just give it a few years, come out of your shell and understand who you are and what you want.