-Is the starter turning the engine over?
-How was the engine running before it failed to fire up?
-Do you get spark when cranking the engine?
It could be a few things (depending on your answers above)
-spark(yes) and cranking(yes) >> probably, main relay (under the dash, right next to driver's side door)
-spark(no) and cranking(yes) >> probably, ignitor or coil (in the distributor)
-spark (???, engine running badly before) and cranking(yes) >> probably, ignitor or coil
It never hurts to check all the grounds: transmission, valve cover, thermostat, car body, negative battery terminal.
_and_ check wiring connections for corrosion, water, dirt, etc.
Take Care,
ChrisS