Well, to get parties without having joined a guild, you have to use the LFG tool and talk to people in your level range and in the area (on higher levels, the later might not be as important anymore). Alternatively one can try a shout in the desired area, or a /who there.
In the endgame, the demand for tank : healer : dps classes is about 1 : 1 : 1. Meaning two tanks, two healers and two dps classes is the standard configuration. The second tank stays in offensive unless multiple opponents attack. This is especially crucial in the important Rhaz Inkur dungeon where CC is completely impossible.
If I might give you some tips, good classes are:
- Paladin, for being the purest support tank and being able to provide some emergency healing
- Dread Knight, for being able to heal himself without the possibility of disruption; especially the mass drain health on higher levels
- Ranger, for being the best bow user as well as a good melee dps as well as being a lesser toolbox
- Bard, for being the perfect toolbox, including reliable invisibility and very high runspeed; also the perfect harvester
- Cleric, for being the defensive healer with good melee buffs and a couple interesting affinities; most popular is Peace with Invis, very fast Root, and extensive Aggro control
- Shaman, coming in 3 variants, each with specific advantages. The Wolf variant, for example, is the only healer class with Stealth, important again in Rhaz Inkur
- Blood Mage, for being full featured healer, toolbox with raid healing tricks, but also downright insane damage output for a healer class if not needing to heal
- Necromancer, for being the only class with fake death that is worth it, being able to tank adds with his summon etc, and besides being the only mage class that can solo well even on highlevel, where mobs can oneshot mages.