Tipping in New York City restaurants is very customary, as it is the main source of income for the waiters. Tipping is usually at 15-20%. It is also often called "a rule of the double tax": multiply the sales tax, which will be specified in the check, by two and get the amount of the tip. Sometimes the tip is already included in the bill - please take a look at the check, it can be called a service charge, or similar.
Sales tax in New York City restaurants is 8.875%. Prices listed in the menu are without the tax, but it will be included in your bill.