I doubt they have a pvp server up for the stress test, but just so you know, the horde should almost always be less populated than the alliance.
And probably the most newb friendly class in the game is paladin since they're similar to warriors in weapon/armor proficiencies (although they don't get any ranged) and they can heal themselves. Warlocks are probably the toughest to get into, and also are going to be polished up a lot in the future, meaning right now they're a bit sucky compared to how they used to be (although quite frankly, what made them good was pretty much a bug). Another class I wouldn't really try is a druid, they're also due for some polishing. Not really a bad class, it's just that the most polished classes in the game right now are warriors, rogues, mages and priests, with paladins and shamans lower (they don't have talents like the first 4 classes I mentioned do, but they're probably going to get talents next).