How is Paladin played today? What, if anything, has changed from the old PLD/WAR flash/voke/cure days?
It's basically played the same way that it was before. You are there primarily in a tanking role and Flash/Provoke are still common ways of building enmity.
What subs besides Warrior will I want to level? I've heard yes and no to some and I would like to level all of the subs I'll ever need to 50 before I start leveling PLD.
Besides Warrior, the good subjobs to have are Dancer, Scholar, and Red Mage. I'm copying and pasting this from another topic where I answered a similar question, and I covered it fairly well over there.
In Legion /SCH and /DNC are commonly used to help with support in Legion, since the Paladin is often on its own to hold the mobs the rest of the group isn't fighting, so it'll need to take care of its own Erases and -na's, not to mention keep up its own healing. /SCH offers more efficiency in MP usage, as well as Erase and -nas, whereas /DNC gets the benefit of uninterruptable Curing Waltzes which can be cycled alongside your Cures to get your HP back up quick, Healing Waltz that can remove most status ailments, and with all the TP you'll be getting from shield blocks (thanks to Shield Mastery), you'll have a decent stream of TP coming back to fuel those Waltzes.
Warrior as a sub is pretty prevalent still for those other situations, for the sake of Provoke and its offensive capabilities. For more solo oriented things and playing around I tend to use /RDM for the Fast Cast and the Refresh, though with Paladin gaining Phalanx natively nowadays, it isn't as potent of a subjob.
What are the Paladin's roles at end game? What is the main role and what secondary roles might I want to prepare for, if any?
Paladin's primary role is still going to be a tank job, and for current events, most of what a Paladin is going to do is holding monsters that the primary DD in your party/alliance isn't fighting. You'll primarily be on your own (as in no support from anyone else in your group) holding the monsters until they're pulled off you. You are essentially just a decoy for the other mobs as the main DDs take out them out one by one.
For example, in the Hall of Muru, the first wave of monsters is an Ixion, an Amphiptere, and a Sandworm. My group will have the Paladin go in to aggro the monsters and pull them to a corner while we pull the Ixion off him. Our focus is on the Ixion and the Pld's focus is on holding the Amphiptere and Sandworm until we get done with the Ixion.
In order to be a successful Paladin, what are the areas I will want to focus my greatest attention in?
Your shield more than anything. Though it can be done without either, most "end-game" shells nowadays will require that any Paladin they bring along has at least an Ochain or an Aegis, if not both. They're far superior to any other shields available, and you'll be hard pressed to be able to get into an "end-game" shell without at least one of them. That being said, the shield that drops from Provenance Watcher, the Adamas
, is a very good shield to have if you don't have either of the previous shields mentioned. It doesn't have the uniquely high shield block rates that Ochain and Aegis have, but its boost to Reprisal allows for a near 100% shield block rate for the duration of Reprisal. It also gives the Paladin a boost to Cure potency received, -5% Damage Taken, and an increased resistance to status ailments.
I've played Paladin in Legion before as an Adamas owner and I've had minimal trouble with holding monsters due to a rotation of Reprisal, Palisade, Reprisal, Sentinel, etc, and deaths I've suffered were ones out of my hand that would have taken an Aegis/Ochain Paladin down as well, so it's not an issue of being significantly outclassed by my inferior shield.
What useful gear can I pick up as a new player to keep me adequate until I have the resources and connections to start working towards more elite equipment at end game?
Just keep fairly up to date melee, weaponskill, physical damage taken, magical damage taken sets, and other random sets and you'll be good. Most gear you'll be using at 99 will be derived from higher level content (Abyssea, Limbus, Voidwatch, etc), so there isn't much anything you can acquire early on that will help you later on, but being able to have multiple gear sets to cover multiple situations, knowing when to swap to those, and being able to on the fly will help you immensely for later on.