Though the ushers at a Broadway theater have many responsibilities---seating people correctly, handing out programs, making sure the show is not interrupted by the audience---they are not responsible for regulating who comes into the theater based on attire. There is no actual dress code for the theaters on Broadway. Many theater-goers decide to dress up in more formal attire to add to the experience. However, if you are not comfortable in formal attire, jeans and a nice shirt will do just fine; you can be fairly certain you will not be the only member of the audience not decked out in your finest attire. Audience members are welcome to wear comfortable, informal attire---within reason, of course. Even in New York City, lingerie and pajamas are not acceptable theater wear.
For better or for worse, a single audience member can influence the enjoyment of a theater production for the rest of the attendees. Therefore, always remain respectful of the actors, the performance and your fellow viewers. In many theaters, if you arrive late and the performance has begun, you will have to remain in the lobby for the first act, or to settle for a seat in the back of theater, so the performance is not disrupted. Courtesy dictates that cell phones and pagers be turned off during the performance or left at home. Laughter and applause is acceptable during appropriate times, but save eating, drinking and talking for intermission or after the show.
Often, theaters will have age restrictions on some or all of the shows. If a child is under a certain age, it is highly possible he or she will not understand that it is necessary to be quiet and courteous during a show. Consider leaving young children at home. If you do bring a child to a performance, prepare them ahead of time that they should be quiet and not disturb those around them or the actors onstage. If a child becomes disruptive, take him or her to the lobby; many theater lobbies include television screens that broadcast the performance.