Idle RPMs are controlled by the Idle Air Controller which is mounted onto the throttle body where it regulates the flow of air into the engine when your foot is not on the accelerator pedal. When it's not working correctly/engine is not idling properly, it generally only needs cleaning. Remove it (two small T-25 IIRC torx machine screws hold it on) and using a spray cleaner like throttle body cleaner or carb cleaner, together with a small brush or rag, clean its solenoide-actuated plunger of all the gunk you'll find on it. Then use the cleaner and a small brush like a baby bottle nipple brush to clean the oriface in the throttle body the IAC's plunger fits into. Once they're cleaned up and the IAC is mounted back onto the throttle body, use the rest of the cleaner to clean up the inside of the throttle body using the directions on the can.
The IAC is located is on the passenger side of the throttle body.