Advanced Placement classes and tests are governed by the College Board. The College Board requires AP teachers to be certified to teach official AP classes. However, students do not have to take an official AP class to register for an AP test. You can study the material on your own and take an AP test from a local high school. High school counselors must order these tests in February, so start calling them in January to ask about reserving your spot. You can expect to pay a proctoring fee of around $20 and a College Board test fee of around $100 per test.
You may take AP classes online from HSLDA or Thinkwell. You can also order study guides and sometimes flash cards for the various tests from Amazon. Each specific AP test is only offered one day a year, in May. They are not easy. They usually have multiple-choice and essay questions. The scores are posted in July. Scores range from 1 to 5, with 5 being the highest. Check with your college of interest to see what scores will earn you which type of college credit. Make sure the credit will be useful toward earning your college degree. Many colleges have lists posted of what type of credit they will give for specific AP tests.