Every time you get on stage it gets a little easier. I'd try auditioning for a few parts that you'd be okay with not getting, or maybe for a school play. It'll be rough at first, but you'll learn to channel your nervous energy in good ways like concentration, voice projecting, etc.
The best way to overcome shyness is by building your self confidence and be an outgoing. You can build your self confidence by having goals, focusing on various interests and most probably taking care of your looks. Visit http://www.casting-call.us/acting-auditions.php to check auditions in your state.