Thee are a lot of components that are part of the air, fuel and emissions management systems that all can cause misfires. It is a process of elimination to diagnose the issue. Attached is a walkthrough which lists components that might be at fault and how to test them in a logical order.
One thing I would reccomend though, have the trouble codes checked at a repair shop or purchase a code reader, as they are geting cheap and pay for themselves quickly, and check the trouble codes yourself. It could shorten the diagnostic process.