I don't know all the flashpoints but the first one is Black Talon for Empire and Esseles for Republic. They are available at lvl 10. Athiss is available for empire at lvl 18. The droids in the starports will have a message for you from Darth Malgus regarding flashpoints.
I completely forgot Hammer Station for empire. I know its before Athiss but I can't remember the level range.