I think that the best way to start acting is to take an acting class. Another way is to simply start auditioning, especially at your school. I know several people who started out performing at my school simply by auditioning for something that was going on. You may want to take classes that are related to acting such as dancing or singing. Just like anything, the best way to be good at something is to start doing it. You may be awful at everything in the beginning, but everyone is awful when they are learning something new.