What races are available to the Horde? Up to the Wrath of the Lich King Expansion pack the horde has access to 5 races: Orcs, Tauren, Trolls, Undead, and Blood Elves.
Once the Cataclysm expansion pack is released it will add Goblins to the original 5 playable races to the Horde.
What Classes are available to the Horde? All ten classes in the game are available to the horde but more specialized classes like paladins are only available to Blood Elves.
You must have a character of at least level 55 to unlock a Death Knight hero class.
Where is the best class to level with? Some classes level faster than other such as pet classes like Hunters and Warlocks compared to a Holy spec Priest that do not run instances, but for the most part every class has its own talent spec enabling it to level .
Best way to level up? Generally staying in an zone that is suitable to your current level and doing all the quests in that zone in the best way to level up your character.
If you have not leveled after completing all the quests in that zone move to another that is around your level or just stay in the zone you are in and grind mobs.
Instances are also another alternative to doing quests and the bosses drop nice items.
Should I spend time leveling professions? Primary professions like jewelcrafting or enchanting can be very useful late game once you hit level 80 but for strict leveling up purposes they can slow you down a lot, once you get the faster mounts in the game gathering materials to level professions becomes much faster, so try to wait till then if you want.
First aid is a profession that should level up at all times.