If you arrive at the concert a little bit early, you'll have time to find out where the restroom is and plan your route. Be prepared to wait in a line at intermission. For some reason, most concert halls don't have enough restrooms to accommodate everyone quickly.
Some concert halls have drinks and snacks on sale at intermission.
Ask an usher where to go. In large theaters there may be a bar, and perhaps even a snack bar. Don't plan on finding a meal at the concert, though. The food on sale is most often snack food and desserts. If you expect a message while you're at the concert, notify one of the ushers.