Warriors: DPS and only viable end game tank, horrible to level because of difficulty and a lot of downtime.
Priest: healer/dps. Shadowpriests can dps quite well during levelling content and they are amazing in PvP, but you will be a healbot in raids.
Druid: Tree-hugging hippies that can do every role to an extent. Most commonly healers though. Very versatile in PvP and during levelling.
Shaman: Tree-smashing hippies that are excellent healers and mediocre dps, but very dangerous in PvP.
Hunter: Legolas with a kitten. Pew-Pew, shoots with bows etc and are very hard to kill.
Warlock: Ozzy with demons. DPS that can choose to use damage over time to do hardly any damage or spam shadowbolts for 3 expansions. Very good duel class + free mount.
Mage: A whimpier version of the Warlock. Can conjure food and water for the raid and can easily grind money.
Rogue: The stereotypical thief, not uncommonly a ninjalooter, the bane of all cloth-wearers.
Paladin: A priest in plate armour and free mount. Impossible to kill, but usually does no damage, unless he reckbombs you.. gg.
Death knight: overpowered as f.. oh wait, wrong expansion.
For more info on the classes, watch this
vidThe easiest classes by far are the warlocks and hunters. They have a pet that can support you while solo levelling and they are easy to play, but hard to master. As for race it doesn't matter that much. Forsaken have the totally OP racial Will of the Forsaken, which is why 90% of hordes are zombies.
I use
these guides for finding all the dungeon quests. Some of them can be a bit tricky, but offer great rewards and XP.
For professions you should always pick up first aid and train it, especially if you're not a healing class. For primary professions I'd suggest two gathering professions, like Skinning and Mining. Crafting professions tend only to favour the end-game raiders who can get the really good recipes and most people will craft your stuff for free as long as you bring the mats.
If you get stuck on a quest you can always go
here to find more information. You can also ask in /general, but people are usually quite feisty and will usually refer you to the location the opposite faction or a world boss.
One last note is that people will often tell you that some specs are absolutely useless, this is not always true. While you can't do much in raids as a protection paladin or shadowpriest, you can do very well in other scenarios such as pvp, solo grinding and the likes.