All valid questions.
Everything I'm about to say assumes you've finished the storyline, and have access to the hard mode primals (quest acquired via the Waking Sands, Urianger I think).
Darklight gear is item level 70 armor and accessories that are purchased via Philosophy tomes, which are dropped starting in the Stone Vigil. You should have over 300 or so by the time you finish all the dungeons leading up to the end of the game (including Castrum Meridianum and Praetorium).
Valor/Cleric gear is part of the class of gear commonly referred to as "AF2" because it looks like the AF (artifact) armor you get during your class quests, only it is much stronger (being item level 90). You purchase this gear by using Mythology tomes, which are similar to Philosophy tomes, but harder to come by. All end game level 50 dungeons drop them with the exception of CM and Praetorium. That means Wanderer's Palace, Amdapor Keep, Pharos Sirius, Hard Mode Haukke Manor and Copperbell Mines, and Crystal Tower: Labyrinth of the Ancients.
Relic weapons (and relic+1/Zenith) are unique job-only weapons that are acquired via a quest that starts in Revenant's Toll, from Rowena I believe. You can only be on one quest line at a time, and to finish your relic weapon you need 900 Phil tomes. To get your Zenith (which is the new name for Relic+1) you need 900 Myth tomes. The quest's themselves are pretty explanatory, so once you get it you should be good to go.
Typically, the way people acquire gear is by farming for Mythology tomes and amassing Phil tomes along the way. They then purchase Darklight armor with the Phil tomes, filling in other spots with gear dropped from the high level dungeons. As time goes by and you accumulate more Myth tomes, you can upgrade your gear further as you see fit.
Hope this helps. It's all a bit overwhelming at first, but you'll get the hang of it in no time.