Educate yourself on the theater. There are plenty of opportunities to take in a local theater production of "My Fair Lady" or "A Midsummer Night's Dream." Or head to New York City and spend some time in the theater district, learning the ropes by learning from the best.
Watch movies that were also plays, and pay attention as if you were the producer of the film. "A Streetcar Named Desire," "Phantom of the Opera", and "Rent" are all plays that were remade as movies. You have the advantage of the remote control to capture a scene to study.
Hit the books. Majoring in theater can help you learn the business of theater while it bolsters your confidence. Taking theater classes in the community is another good way to learn about the business.
Be professional. Leave a good impression on everyone you meet. Proofread all your written correspondence before sending it to a prospective reader of your work.
Read your play, and then reread it. Have a group of others read it, and revise it from there. When you've worked out the bugs, you can then think about shopping it around.