what should I test and where do I order needed parts?

 

Of course try a new spark plug before anything else. Next would be to remove the kill circuit wire(s) (at the coil) and check to be sure you have proper ignition coil to magnet clearance (typically .010-.012") and then test for spark. If no spark then you need to replace the coil. If spark then it is highly likely you have a defective kill switch but you may have a bare kill wire. You can test the kill switch with a ohm/continuity meter to confirm. Do you have a local Stihl shop you can buy the parts - if needed - from?