The rule used to be $10 per hour if you have a bachelors, $15 per hour if you have a masters, and $20 per hour if you have a PhD. However, this was 15 to 20 years ago. The rates may have gone up by now; but I think this still sounds fair.
I would charge at least $30 an hour. If it is a more difficult subject, such as ochem or calculus I would charge $50.