Theres a lot that could be wrong, heres a few things to check :
Spark plugs and wires, distributor cap, fuel filter, carburator.
I'd definitely start with a can of carburator cleaner (you should be able to take the air filter off and just spray it in there) the stuff takes the gunk right off and helps your engine run better. Just make sure your engine's running while you do it, and spray a little bit at a time.
good luck