[Mardur]

If you want to easily find groups, pick a class that easily fits into one of the three archetypes:

Healer: Your only option really is cleric. Shaman can easily get groups at the higher end, but they take some time to come into their own. Most groups fly without a shaman, very few groups fly without a cleric. It's also a bit easier to learn/manage a cleric than a shaman.

Tank: Your choice between warrior and hybrid (SK/Pally). Other than the experience penalty, hybrid tanks are more friendly to newer / casual players. This is because warriors have to manage their money if they have no start up funds, and do some searching for gear, especially early aggro weapons (such as obsidian shards). Hybrid tanks have the convenience of generating aggro through spells, so you have a better chance of not disappointing a group.

DPS: At the lower end, most people will want a caster such as a magician or necromancer. This is because the pets do very good damage compared to the low end weapons most people will be carrying. At the high end however, rogues never have trouble finding groups. Monk is also viable, but you'll be expected to pull which is a special skill that has to be learned/practiced.

TLDR: For a first time player who wants to be welcome into groups, figure out what role you want to take. If you want to sit back and buff/heal, choose cleric. If you want to be tanking some mobs in the face, go with SK or Paladin (possibly warrior). If you want to avoid and deal damage, go rogue.