I have my own tutoring business working primarily with high school students. I love it. It gives me the flexibility to be home with my daughter while making a nice income. I live in the DC area, so tutoring can be very lucrative. That being said, the rates really vary based on experience, whether you work independently or for a company and what areas you specialize in.
If you want to do homework help and academic work, you can work independently utilizing your school contacts, etc. If you want to do test prep, I would recommend starting through a well-established company like Kaplan or Princeton Review so that you get the proper training to be successful.
Most tutors work between 5 and 15 hours per week as a second job. If you are an experienced teacher, I would not tutor for less than $25/hour. The average for a new tutor working for a company is between $20-30 to start in our area. As I have been doing this for many years, I make substantially more. You can charge more as an independent tutor, but it is a challenge getting new students versus having a company to recruit students for you.
Feel free to PM me if you have other questions.