How do I determine the most probable cause of a starting problem?

when the car is in a crank-but no-start condition , turn the key to 'run' position and watch the check engine light and listen very carefully for a light click when the light goes out. try several times to verify. IF you do NOT here a click when the light goes out , then it's a good chance you have a failing 'main' relay. IF you do here a click , then you need to check further. Vehicle mileage and maint. history would be good info for us , to start.

This is a very open situation. Feed us some more. However, you can have the the starter checked at the parts store for free.

agree with ziptie

agree with #2 also