You tend to find out about flashpoints on the planets. There's generally a droid that gives out missions. If your desperate to play one, and find out the levels etc, I'd suggest going to the fleet. There you can pick them up, and check them in your quest log to see what level they are. You can first do flashpoints, when you first leave your starter planet.
Edit: In terms of the order... not many people know, you learn that as you go along...