How does a girl have sex on top of a guy?

Hi,
I think the best place to learn about this information is the bookstore! There are many great resources that can illustrate the "woman on top" position much better than we can even describe it.

Many of the human sexuality books are meant to educate (as compared to pornography), and you could look through a College Textbook on Human Sexuality for different sexual positions, or a book such as The Joy of Sex (or another modern book) would provide enough images and text to walk you through the process.

Don't be ashamed or scared to look through these books, so you can learn techniques and also how to have sex that is protected and safer (with the use of birth control, including condoms).

Lastly, if you are in an intimate relationship with someone, enough to have sex, I hope you two are also intimate enough with each other that you can talk about different sexual positions, help guide each other lovingly into different positions that feel mutually pleasurable, and have fun by experimenting together.