Tipping your server in a restaurant should always be a matter of personal choice, but it's a good idea to tip a good waiter or waitress; they don't get paid much. The standard tip varies from 10-20%, but 15% seems to be the most common accepted rate.
Some restaurants in America, and many restaurants in other countries, often include a 15% tip directly on the check. Keep an eye out for this practice, so that you won't accidentally double-tip. While the wait-staff won't mind, your pocketbook probably will.