First, thanks for putting in some effort to look around and see what it is pld can do. You'll meet quite a few players in game that think they completely understand this job and what all it can and can't do but as I'm sure you've probably seen in some of the newer threads here, it's not always the case.
You can tank anything as pld/war but the work required by everyone else to make pld/war feasible for some things is quite huge and often times not worth the trouble. To lessen the burden on others, a pld uses /nin to blink away as much dmg as possible for some things. Examples here would be Fafnir, Nidhogg, Adamantoise, Aspidochelone, the sky guardians, and many sea trigger mobs, jailers, and other things. Blinking away dmg also plays part in low numbers situations, like certain NM fights, like those campaign op bc, some ksnms, MMM, limbus, etc. For meriting you can use /nin not only for the dmg mitigation of utsu but also for dual wielding swords, of which pld has access to two swords that can attack multiple times per round (quite fun to use).
/war comes into play when you'll want a quick punch of hate against ever changing mobs, like dynamis, some limbus floors, exp, merits (not so much to tank but for berserk, DA, and also voking mobs off puller), and trigger farming for some things (like in sky).
/rdm has some uses for a few situations where the monster either doesn't melee attack you constantly or doesn't get to attack much at all. Jailer of Love is a big example of this since you can tank it from a distance. Some manaburn strategies can also work this way, such as ouryu commeth, where the mob is slept when on the ground and nuked when it's in the sky. /rdm in these situations is superior to other choices because you gain constant hate tools in the form of spells that will keep your hate capped on the monster. It's use for exp and merits is limited at best, but for solo work it's incredible with the use of phalanx spell, enspells, and stoneskin.
In all, don't worry about endgame right now when you're just starting out. In fact, leave that for after you hit 75 and are comfortable with the time required for some of those events. Just play the game, make as many friends as you can, and leave good impressions all around. It will only help you later on since you're much more connected to your character's name in ffxi then in most other MMOs.