Could be a few things:

1. Dead cell in battery keeping you from getting enough charge to the starter.

2. Bad Bendix

3. Bad Starter

It takes much less juice to get your lights on than it does to get the motor started. Try jump starting it and see if it starts up as normal. If it does, it's probably the battery. You can take it to Autozone or Pep Boys and have it tested. My OEM battery had a dead cell at 33k miles.

-- John