Inquire at local and national agencies, as well as with independent tutors at different levels to figure out what you're worth. If you market yourself professionally, you can command a premium, especially if you can provide evidence of strong teaching credentials or a prestigious education.
Adjust your rates according to your demand. If you start out too high and notice that nobody is responding, or people are saying you're too expensive, lower your rates. If you're charging so little that demand is overflowing, raise your rates. Eventually, you will find a comfortable point where you are being rewarded fairly and getting a steady flow of new students.
Charge different rates depending on the amount of preparation time required, the amount of skill required, and the perceived importance of each subject. Tutoring for AP courses and the SAT, for example, is typically charged at a higher rate than other subjects because these require specialized knowledge and are considered very important.