How to do girl on top? help!?

Do whatever is comfortable for you. You can sit straight up and slowly move up and down his "thing". That will usually make your legs tired so then lay with your chest to his and move up and down. I like it this way better. Sometimes you can lay flat and just move your hips up and down. Don't think to much about his pleasure (that may make you a bit nervous) just think about what is feeling good to you, try to close your eyes and focus on whats going on down there. You will learn together and it will be different each time. Tell him that it is your first time and to sit back and let you "play" till you find what you like. If he loves you (and he damn well better if you are doing this for him) he will be happy to oblige. I PROMISE if you like it, he will LOVE it. God I hope you are at least 17!