Acting...theater classes?

I want to try an persue acting, im a teenager still under 18 in high school. as a young kid i was in like 4 plays through a local theater class (i didnt audition i signed up for the class) but had small roles everytime. im interested in trying extras casting in LA, because i live about 2, 3 hours away from there, but i want to be more experienced before i sign up. so im thinking of signing up for outside of school theatre. is it neccessary to have experience with theater to do extras casting and acting, and do i need to be a good singer to be successful with theatre? im not the best singer.