Other contributors have said "How did Steve Carrel become an acter?" is the same question as "How did Steve Carell become an actor?" If you believe that these are not asking the same thing and should be answered differently, click here
Perform in plays, memorize lines, be able to add emotion to the lines. Eventually, if your talent is recognized, you just might have a shot at the job of an actor.
Every actor starts as an unknown, and as such, you take whatever acting roles you can get - starting with school plays (if you are still in school). Any acting that you do will help you to learn...