Arrive on time. Be 10 to 15 minutes early even. At some theaters you will be denied admission if you arrive after the show has started.
Use the restroom, drink water, make phone calls or whatever other personal needs you might have before the show starts. It is rude to leave the show during the performance. That's what the intermission is for. Also turn off cell phones or pagers. Don't just put them on vibrate, turn them completely off, or at least go to silent mode.
Sit in the appropriate seat. If you have a general admission ticket you can select your own seat. If you have an assigned seat number, sit where you are supposed to.
Read the theater program so that you have some idea what to expect. This sense of structure is particularly useful if you have never been to the theater before. Also, the program is a great source of information about the production and its participants.
Watch and listen to the show quietly. Respond appropriately to moments during the show, such as funny scenes. Do not talk or whisper during the show. Do not open food wrappers, or loudly slurp drinks. Some theaters don't allow food or drink.
Return to your seat promptly after an intermission. Also, be sure to turn your cell phone or pager off if you have one on during the intermission.
Applaud when the show has ended. Remain in your seat, unless a standing ovation is called for. The actors will return to the stage so that you can applaud them directly. Sometimes an encore is called for, especially in musicals, so be sure not to leave until the house lights have come on and you know the show is completely over.