Dirty throttle body, sticking idle air solenoid, low fuel pressure?
Could be really bad gas or a vacuum leak as well.
Is the check engine light on?
Take a rag and wipe out the throttle body around the throttle plate really well and make sure the air and fuel filters are OK before getting to involved.
I have to clean my throttle body out about once a year and I keep an eye on the air filter as well. They last me about 2 years. The same for the fuel filter but with the poor quality of fuel lately I think the fuel filter may be getting changed a bit more often.