You need to clean the throttle body and butterfly flap inside the thottle body if this does not correct the problem you have to replace the Idle air control valve.
If you post back what engine you have I can post more information ie: component location and R&R instructions.
You can clean the throttle body with carb cleaner and a bottle brushjust have someone hold the gas peddle to the floor while you clean it.
The air bypass valve or Idle air control has 2 screws holding it on be sure to change the gasket if you replace the valve.
If you want still more Info please post back with your questions I will be happy to help.
Have someone press the gas peddle to the floor and using a can carburator cleaner (aerosol) completely clean the inside of the throttle body and both sides of the flap when you look inside the throttle body it will appear obvious what you need to do.
Use carburator cleaner and a bottle brush, it will clean up easily and should only take a minute or two.