What all should I check?

Plug
Plug Wire - Can I test this? How?
Coil Pack - Can I test this? How?
Injector - ...

If I change the plug and it still doesn't fire, then what should be my next step? What actually drives the firing sequence? Everything else seems fine.

Thanks.

 

Thanks. It was a bad plug. They told me they changed them out! You never know.