Decent classes for beginners;
Druid, Paladin - Those are quite versatile, and will give you a fairly easy time while leveling. Seeing how both can also fulfill any role in PvP and PvE, you can try a lot of different things and see what you like the most.
Hunter, Warlock - Both use pets to soak up damage and tank for them, making it fairly simple and safe classes to level up with since you're standing a mile away from danger shooting arrows/spells at the enemy.
Less decent classes for beginners;
Mage - At lower levels (up to 30-35) you'll run out of mana all the time, so you'll spend a lot of time replenishing mana (doing nothing while your character sits there drinking water) It gets better at higher levels, but chances are you won't be patient enough if it's your first.
Warrior - A class that depends on equipment a lot, which is hard to keep up to date while leveling, especially when it's your first character. At higher levels this can become a pretty complicated class to play, lots of fun, but it needs lots of work too.
Other than that;
Shaman - Starts out fairly boring, but slowly gains more and more tools, becoming more fun at later levels. Though all the different things could be overwhelming at first.
Rogue - Really just needs two proper weapons to function, if you get the hang of it fast it's pretty easy to play, but if you don't, prepare to die a lot. Pretty repetitive qua leveling.
Priest - I honestly haven't leveled a priest, but friends who have were often bored while doing so. Especially at low levels you'll spend a lot of time and mana per enemy.
General tips;
-Thottbot and Wowhead are your friends. These two sites have just about all the info you'll ever need about your character, where to level, what to do next, etc. etc.
Though I wouldn't visit these sites for the first few levels or so if you like figuring things out for yourself.
Social:
-Be polite and respectful to people, they really will like you for it.
-If people are being rude to you for any reason, just ignore them.
-Use grammar and spelling. No one minds a typo, but 'txt language' is generally associated with stupidity.
-Don't ever beg for money, items or equipment, it is extremely annoying.
-Don't spam in any chat but Guild Chat. It's annoying and you'll end up on the ignore list of decent people that you might want to play with.
-Even if others are doing it, and it seems funny at the time, don't spam.
-Seriously, don't.
-If you're lost, read the quest text. If you're still lost, ask politely.
Enjoying the game:
-While leveling, try to find a fun leveling/social guild, you'll have people to help you do hard quests, give you advice, or just randomly chat and have fun with.
-Playing with friends makes the game a lot more fun, if you don't have any real-life friends on your server, try making some friends to play with ingame.
-Having someone to level with makes everything a lot easier, and more fun.
-You will run into jerks every now and then. Don't let it get to you.
Misc:
-Never be afraid to admit that you don't know something. Most people won't mind explaining a boss beforehand, but they will hate you if you kill off the party because you didn't know the tactics.
-It's allright to ask a high-level player for advice, but sometimes they might be too busy to have a chat, respect that and they'll be more likely to make time for you later.
-If you're planning on making a Death Knight, don't, or at least level a different character to 80 first. I won't go into detail as to why, but you'll see.
-Pick two gathering professions (of which one is skinning) and put everything you find on the auction house for some extra income. You'll have plenty of time to focus on other professions later.
-Have fun. Remember that no matter where you end up, doing what. Don't let it end up your second job. You're not playing a game to please others or kill time, you're playing to enjoy yourself,