Question1: all the classes kind of build off one another, Basically to be a decent you'll need to lvl all the battle classes to something, it's usually pretty apparent. Like in the early days it's wise to take CNJ to at least 6 for protect, and PUG to 6 for Second Wind, when soloing these are indespinsible and for a Tank Second Wind is a must have skill. at the same time, just about every class benefits from having Sentinel... Basically....what I'm saying is....you'll wind up playing every job and you may come away finding out you like jobs you thought you wouldn't. I was pretty much a melee DD person but found I really dig THM/BLM after just trying to get a specific skill from it, and also thought I would hate ARC, but that's now basically my main, and I only started lvling it for Chameleon.

Question 2: You have to do the first 6 quests for all the GC before you can officially join anyone of them, you'll usually walk away from the Join up process with around 1.5k seals (give or take) for each company. when it comes time to join ONE AND ONLY ONE, spend the seals from the companies you don't want to join, as they will be locked (not deleted, but might as well spend 'em) and then Join the one you like best. the rewards are comparable across the board so choose the one you like, not the one with "best rewards".

Question 3:...fairly similar but no Dungeon Finder (yet), but mostly a run through trash mobs to get to a boss... so yeah, fairly MMO standard.