Your throttle body is probably dirty,The throttle body has a hose from the air filter housing attached to it,the throttle body is attached to the intake phlenum.Take the hose off(its rubber with a clamp on it).Do this with a cold engine,NOT RUNNING ! spray some carburator cleaner inside the venturi and scrub it with a toothbrush,on the side of the throttle body there is a place where a cable goes into,twist the direction of cable to open the throttle plate(inside the throttle body),and continue to clean.