In general the Alliance has double the amount of players than the horde. Yes there are servers where horde is greater in numbers, but those are few and far between. But a lot of then are young kids and maturity may become an issue, but we have not found that too much.
The Horde tend to be more mature players, more organized for pvp (horde always wins pvp for some reason), However due to the low population of horde on a lot of servers end game progression is really only obtainable to a very few guilds, where on the alliance your chances to get into a end game guild is much greater due to the numbers.
As for race and class, you can read about each class and pick one that sounds interesting to you, race doesn’t really matter a whole lot, but check out the racial talents from each race and see what one fits the class you want to be.
Let's say you want to play a Warrior, and you want to find a good race for the warrior since Warriors need strength the Orc's racial called Bloody and Fury makes you go into rage and increases your attack power (Strength is converted to attack power), and the orcs also have a axe specialization to give you a bonus when you wield an axe. But the Human is also good for a warrior to since there Racial is Every man for himself which makes you escape from speed altering effects or traps etc. and it could've prevented you from damaging your enemy without that racial. But there a trinkets that people buy from a PVP Vendor that is like the Every Man For Himself that takes a spot in your trinket place since you can only equip 2 trinkets so since you have to equip it you'll only have 1 trinket you can get that has like Strength, Stamina etc. So it's your choice.
After reading this, have you make up your mind to choose the faction of your character? If not, you can take a visit to some professional sites to get some more information. (btw, I want to tell you that http://www.mmocart.com and http://www.wowwowgold.com are helpful)
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