Generally, a tip may vary between 15% to 20% of your bill, or even more regardless on number of people in your party. Depending on the situation, some restaurants automatically impose a minimum amount of tip when they suspect the customer is unaware of the custom. This may also occur when the restaurant finds that its patrons frequently do not leave tips for waiters and waitresses. Some restaurants also automatically charge 18-20% or more on parties of four or more. It depends upon the situation and the restaurant.
All of these rules are to prevent waiters from being short-changed by customers who are unaware of what is expected of them.
A general rule of thumb for tipping is to always leave a tip if you are being served by a waiter or waitress, but you should always check on your bill in case a tip was already automatically added to avoid double-tipping.
You may be surprised to know that waiters at restaurants typically get paid much less than the federal minimum wage, often about $2.13/hour. Most of their salary comes from your tips.
Here are a few articles on tipping that can help clarify these confusing rules: