Horde or alliance

Answer 1

There are a number of things that can influence your decision whether to go Alliance or Horde, as below:

Style

On the whole, Alliance presents itself more as a regimented, clean-shaven force (except the Worgen of course) and comprises of classical "good-aligned" races such as Humans, Dwarves and Elves. The Horde is a gathering of several more feral races, such as Orcs, Tauren and Trolls, who generally band together because they are threatened by the Alliance.

Server

You can check on a server's realm forum whether there are more Alliance or more Horde on the realm. If you prefer to have a lot of support, big guilds and more items available on the auction house, go with the faction that is in the majority. If you like a challenge and playing the underdog, go with the other one.

This is especially true (and important to check beforehand) if you roll on a PvP server, where players of different factions are free to attack each other.

Game mechanics

This is a bit different - while every race's racial abilities are useful in some way, some classes and races mesh really well. An Orc has a bonus on pet damage and can increase its spellpower/attack power - making them natural warlocks and Unholy death knights. Humans gain a bonus on Spirit and expertise with swords, meaning they make good Paladins (Holy/Retribution).

For more details on this per class, I'd suggest finding out what class you want to play, and what race fits with it - and remember to also pick a race that you find fun to play. When you know what race you want, that also determines your faction.

Friends

If you already have friends on a server, you may want to make a character of their faction, because you cannot talk to people of the opposing faction. You also won't be able to group together or join guilds together. So if you roll a character to be with your friends, make sure to be the same faction.

In the end, the choice of faction itself is not all that important, but the factors above might help in making the decision.

Answer 2

Doesn't really matter. Each side has its' strengths and weaknesses.