If your resume lists sufficient credentials (high SAT scores or gpa, attending or graduated from a good college, or previous tutoring experience) it's easy to get a job at a tutoring agency. They take a 50-70% cut of earnings, leaving you with $11-35/hr. There are many such agencies both for in-office and in-home, mostly 1-on-1. Agencies exist that focus on test-prep (preparing for the SAT, mostly), that focus on supplementing an existing class, or both. If you don't have the credentials, it's still worth a shot to apply.

Alternatively, you can advertise on WyzAnt, University Tutor, Craigslist, or put up fliers in the library and high school student centers. I never had the courage to put up fliers but I saw many, and the more experienced tutors I knew did this.