Ok, to begin with, there's really two (kinda three) 'legacies.' The novels are, like, 80 years before the comics. I can't say which novels are worthwhile since I've literally only read part of one.
Of the first legacy comic, Red mentioned the key plot stories, though there is *plenty* of good stuff aside from that- I like pretty much any part that deals with the Alliance and Gar Stazi (Admiral Badass!), for example, and ones starring minor characters, like 'Noob,' the story of a Storm Trooper under the sith. Reading through the whole thing is a quite reasonable course of action.
Starting with Broken is pretty much required in any case, since it tells you Cade's story in a nutshell.
Then there is a *second* legacy comic, a smaller scale one focused on Ania Solo and an Imperial Knight. It's also good, but it's fairly short, and doesn't cover the world as widely as the first comic does. Story wise, it very much comes after the first one, so only read if you want to see part of the aftermath of War.