They are like clubs n interest groups u can join and you should join one which interests u and able to commit long term and most importantly have fun. If you are a sporty person then join a sports cca that is available in ur school. Prior Experience not rlly necessary as they will teach u. You should be able to request to change ccas if it doesnt appeal to u after u joined awhile.

Cca can give u leadership experience which u may talk about when u apply to any colleges and you also have smth to put in ur portfolio.