While you cannot select any race / class combination you want in World of Warcraft because Blizzard has imposed some limitations, you can make some bad choices.
Any combination you are allowed to pick will work fine but, there are some combinations that can really enhance your game play as your character levels up.
On the Horde side choosing something like a Troll for a hunter because they have increased skill with ranged weapons makes sense unlike a Human Hunter on the Alliance side because there are no real racial attributes of a Human that make sense with the Hunter class.
Well it work fine as a Human Hunter? Yes and you get things like reputation gains but, A Dwarf that also has increased skill with ranged weapons may have a leg up at higher levels.
Learning how to choose a race in WoW is also about what you are going to be playing with that character. PVP players look for different attributes than those interested in PVE play.
Players choosing spell casting classes on the Alliance side may favor Humans because of the increased spirit or a Draenei because of the racial that provides a better chance to hit with spells. There is also the option of Worgen that offers increased critical hit chance.
Looking at the PVP environment it has always appeared that most Horde races have an advantage because it does appear that they have more PVP related racials than Alliance. This has been debunked over the years by showing the true balance between Horde and Alliance on the Battlefield and in the Arena environment.
The bottom line is that players should spend some time considering the impact of the race they choose in relation to the class they want to play. If you are looking at becoming a serious player in the PVP or PVE environments it can certainly make a difference at the highest levels of play.
If you are simply a casual player that wants to mess around and have some fun, pick a race you like to look at because you are going to be spending a ton of time looking at your character as you play the game.
I have always been a fan of power leveling different characters and I usually try some odd race / class combinations just to see how they work out.