you should ask your high school counselour or some teacher at your high school or u can go to a college and just ask the just say thaht u are in highschool and u want to earn college credits
If you have a local college that offers online classes, you should be able to sign up for them. But you may have to get special permissions ahead of time.
Just as an FYI - I took both online and in-class courses at a college about 5 years ago. I enjoyed going back to school very much, but I also learned something else. The online courses took waaaay more out of me than the in-class ones. You’d think it’d be easier, but really it wasn’t.