Freelance work, how do you charge?

Hey guys. I have been doing a bit of freelance-ish work foor a person i know, which I'm getting paid for.

It's a simple cms solution in Ruby on Rails(Bite me, it's an awesome framework and Ruby is fairly nice), with a login page and then it just replaces the labels with a rich text editor, and stores the html in the database.

I didn't really take note of how much time i*spend making it, nor do i actually know what to charge per hour.

Since I'm a student(17 years old),*I can't charge that much.

So what would you guys charge? I'm looking to have a few tasks like this in the future, so it would be useful to know.***