Clean out the throttle body with fuel injection cleaner then check your vacuum lines if all are good then you should have codes set your check engine light should be on verify that the check engine light bulb is not burned out turn the key to the on position you should see all the dash lights //oil//battery//check engine light if you don't see it the bulb is blown out and if all other things I mentioned are done and the problem is still there either your throttle position sensor is bad or your idle air control valve but I would go to the throttle position sensor first because your car is now 17-18 years old by the door sticker date and if you don't know the repair history because it's a used car check that sensor first if you can get a code scanner from someone/ loaner from autozone you will no for sure