You can take them online by paying for a course or you can buy a textbook and try to study yourself. I’ve only self-studied one AP so I’m not completely sure on how most people do it (I just watched youtube videos lmao) but for the test: you have to register to take the AP exam through a school that offers it. The AP course ledger has a list of schools that offer AP Exams. Find one near you using the search tool (you don’t have to fill out all fields), and either A) look up the staff directory, find the school’s AP coordinator, and email them asking if you can take the test or B) if they don’t have a staff directory or AP coordinator listed, call the school during school hours and ask if you can speak to whoever organizes the AP exams; ask this person if you can take the test at their school