Ok, woah, back up.
First, before you choose the race, make absolutely sure on what class. Ill go through a brief runthough of the classes.
Warrior: A bit hard at leveling, seeing as you have no healing or escape abilities. You will most likely tank with groups, meaning you hold the monsters while others do the damage.
Rogue: Damage dealer. You can stealth (cant see you). Easy leveling. Youll be doing damage in groups.
Warlock: My personal favorite. Warlocks are largely self-sufficent. They also get a pet. Very easy leveling. Youll be excellent at soloing, and youll be doing damage in groups.
Druid: The jack of all trades class. Any type of druid is good at leveling, but the over or understatement of this varies. Depending on what type of druid you are, you can be healing, tanking, or doing damage for your group.
Priest: Depends on what type you are. Anything but shadow priests are hard to solo. Shadow are moderate to easy soloists. Hard leveling. Youll most likely be healing in groups.
Shaman: Depending, moderate to easy leveling, based on what type of shaman you are. You can be healing or doing damage in your group. Again, soloing depending on type.
Paladin: Excellent soloers. Another jack of all trades class. You can be tanking, healing, or damaging for your group, although most players are biased pn paladins ability to do damage.
Hunter: Very easy soloing, because you get a pet. Poor damage in instances, since most groups wont want you to use your pet.
Mage: Depending on type, moderate to easy leveling. Mages are high damage dealers, ans specialize in both high damage single hit spells and area of affect spells (hit multiple targets with 1 spell). Youll be doing damage in groups.
Ok. Now. Your race is based off these, because each race has a different set of racial abilities. Undead are good all around, due to Cannibalism (eat humanoids and undead to restore health) and Will of the Forsaken (become immune to certain spells for a short duration) and they get more intellect than other classes (mana). Tauren get more health than other classes, and they also get a small stun spell, good for escaping, or casting a healing spell without being interrupted. Because of this, they make both excellent healers and excellent tanking classes.
It is largely your choice, and which you prefer, but if youre going for efficency, Id go undead, because their racials benefit any class.
But still, dont rule out the other races. Trolls get berserking (faster attacking and casting speed for a duration) and health regeneration.
Orcs get bloodfury (higher damage for a duration), stun resist, and increased pet damage.
Blood Elves get a mana drain and an interrupt spell.
Alliance, you have Humans, Night Elves, Gnomes, Dwarves and Draenei.
Humans get perception (easier to see stealth units), reputation increases, and easier to use swords and maces.
Night Elves get shadowmeld (stealth, but you cant move), and wisp form (move faster in ghost form).
Dwarves get treasure finding, stoneform (increased armor, immune to poison and bleed for a duration) and easier to use guns.
Gnomes get increased intellect, and the ability to get out of rooting spells (where you cant move).
Draenei get an excellent heal spell that doesnt cost anything, and a +1% to hit chance that affects your entire group.
Each class does better with different races. Listen, personally, Ive started over countless times cause It took forever to find the combo I wanted. Think hard on this, as it will determine your entire WoW experience. If you ever play on server Andorhal, alliance side, talk to Daxz. Thats my character, and Id be glad to help you out and point you in the right direction, but if you still wanna play horde, I wish you luck, and good fortune.
Laterz.