Oh, and about being a healer. I'd recommend against leveling as a healer. It's slow, and since you gain experience by killing things and questing (which pretty much unanimously consists of killing things for various reasons), healing gimps your leveling speed. Once you get 40, if you're really good at making money, or you have rich friends, then you can dual-specialise.
Also, keep in mind that every class has three different talent trees. I'll break them down for you.
Hunter:
-Marksmanship. This is based around ranged weapon damage and hard-hitting damage. Solid tree.
-Beast Mastery. This focuses on buffing your pet (and yourself) to make it a very dangerous piece of work. Great for leveling, and good for PvP/regular DPS.
-Survival. I wouldn't recommend it for leveling, but you can make it work.
Warrior:
-Protection. Tanking. Enough said.
-Arms. DPS.
-Fury. DPS.
Warlock:
-Demonology. This is based around your demon minions. It gives you an extra one, and it provides you with bonuses depending on which minion you have out. Good leveling tree when you unlock the new minion.
-Affliction. Based around 'curses' and DOT (damage over time) spells. Good damage.
-Destruction. As the name suggests, this one is based around nuke spells.
Mage:
-Fire. DPS. Has Pyroblast, the 'kamehameha' of the WoW world.
-Frost. DPS, but with a large amount of utility. Slows enemies down a lot, which makes this beautiful for PvP.
-Arcane. Honestly don't know that much about this tree. It's DPS, but I think it may be used primarily in conjunction with other trees. Don't quote me on that.
Priest:
-Holy. Healing. I don't really need to elaborate.
-Shadow. Priest DPS. Good class with some funky spells later on.
-Discipline. Healing also.
Paladin:
-Holy. Healing.
-Retribution. DPS. Pretty good DPS at the moment.
-Protection. Tanking.
Shaman:
-Elemental. Caster Shaman. Not bad.
-Enhancement. I'm not too familiar with this spec. I think it focuses more on melee-central damage.
-Restoration. Healing.
Druid:
-Feral. Now Druids are an odd class. They pretty much have 4 specs. Feral is divided into 2 parts, which often overlap. Cat, and Bear. Cat is feral DPS. This will be what you level as for the most part (until level 40 if you go Balance). Bear is feral tanking. Cat is probably one of the hardest DPS classes to pull off at level 80, but it can be incredible. Bear is a solid, respectable tanking build.
-Balance. Druid caster form. Not that great (but still alright) until level 40, where you unlock Moonkin form (affectionately nicknamed 'Boomkin'). This owns.
-Restoration. Druid healing. At a higher level (forget which), you unlock the tree form. This buffs your healing like crazy, but you move slower. Good thing healers don't have to move that much.
Rogue:
-Subtlety. Stealth based.
-Assassination. This is more burst damage. You pop em quick and clean, but in an extended solo fight, it isn't as good.
-Combat. Sustained DPS.
Death Knight (Note: You can't make a Death Knight until you have a level 55 character).
-Blood. Primarily a DPS spec. You can make a Blood tank, but it's more of a niche build than Frost. Blood DPS is high damage, well sustained, with lots of self-buffs and life-stealing to keep you in the game. Supreme for leveling.
-Frost. Primarily tanking, but you can DPS with it, I guess. Never really tried doing that.
-Unholy. DPS. It all comes down to personal opinion, but I prefer Blood. Unholy has ghoul minions that help you, and it works on diseases and stuff. Neither is really better than the other.
Also, keep in mind that every class has three different talent trees. I'll break them down for you.
Hunter:
-Marksmanship. This is based around ranged weapon damage and hard-hitting damage. Solid tree.
-Beast Mastery. This focuses on buffing your pet (and yourself) to make it a very dangerous piece of work. Great for leveling, and good for PvP/regular DPS.
-Survival. I wouldn't recommend it for leveling, but you can make it work.
Warrior:
-Protection. Tanking. Enough said.
-Arms. DPS.
-Fury. DPS.
Warlock:
-Demonology. This is based around your demon minions. It gives you an extra one, and it provides you with bonuses depending on which minion you have out. Good leveling tree when you unlock the new minion.
-Affliction. Based around 'curses' and DOT (damage over time) spells. Good damage.
-Destruction. As the name suggests, this one is based around nuke spells.
Mage:
-Fire. DPS. Has Pyroblast, the 'kamehameha' of the WoW world.
-Frost. DPS, but with a large amount of utility. Slows enemies down a lot, which makes this beautiful for PvP.
-Arcane. Honestly don't know that much about this tree. It's DPS, but I think it may be used primarily in conjunction with other trees. Don't quote me on that.
Priest:
-Holy. Healing. I don't really need to elaborate.
-Shadow. Priest DPS. Good class with some funky spells later on.
-Discipline. Healing also.
Paladin:
-Holy. Healing.
-Retribution. DPS. Pretty good DPS at the moment.
-Protection. Tanking.
Shaman:
-Elemental. Caster Shaman. Not bad.
-Enhancement. I'm not too familiar with this spec. I think it focuses more on melee-central damage.
-Restoration. Healing.
Druid:
-Feral. Now Druids are an odd class. They pretty much have 4 specs. Feral is divided into 2 parts, which often overlap. Cat, and Bear. Cat is feral DPS. This will be what you level as for the most part (until level 40 if you go Balance). Bear is feral tanking. Cat is probably one of the hardest DPS classes to pull off at level 80, but it can be incredible. Bear is a solid, respectable tanking build.
-Balance. Druid caster form. Not that great (but still alright) until level 40, where you unlock Moonkin form (affectionately nicknamed 'Boomkin'). This owns.
-Restoration. Druid healing. At a higher level (forget which), you unlock the tree form. This buffs your healing like crazy, but you move slower. Good thing healers don't have to move that much.
Rogue:
-Subtlety. Stealth based.
-Assassination. This is more burst damage. You pop em quick and clean, but in an extended solo fight, it isn't as good.
-Combat. Sustained DPS.
Death Knight (Note: You can't make a Death Knight until you have a level 55 character).
-Blood. Primarily a DPS spec. You can make a Blood tank, but it's more of a niche build than Frost. Blood DPS is high damage, well sustained, with lots of self-buffs and life-stealing to keep you in the game. Supreme for leveling.
-Frost. Primarily tanking, but you can DPS with it, I guess. Never really tried doing that.
-Unholy. DPS. It all comes down to personal opinion, but I prefer Blood. Unholy has ghoul minions that help you, and it works on diseases and stuff. Neither is really better than the other.