Two of the most common problems are a dirty throttle body or a bad idle air control (IAC) valve. The throttle body is easy to clean with cleaner made for the purpose. Most auto part stores will have this in stock and simply follow directions on the label.
The IAC is a cylinder looking device, with two hold down bolts and a wire, located near the throttle body. To test, try tapping gently on it with a screwdriver handle. If the vehicle temporarily starts to idle the IAC is likely bad.