Each class is able to choose from two different specializations swtor credits eu at level 10. You should read both of them carefully and even use outside websites/sources for information. The reason you want to make sure you are making the right choice is because once you have made a choice you can't go back. If you are playing a Trooper and you choose Commando over Vanguard by accident you will need to start a completely new character to fix your change.
I can't stress the importance of researching this before the game comes out so you know exactly what you're going to do. As for a bit of self promotion; a good set of class guides to read in helping you choose your class are right on my website actually. You can find them in the Getting Started section or links right now, depending on which allegiance you want, The Republic or The Empire. Keep in mind that while all of the classes are named differently and look different on both factions, the same mechanics are behind all four of them.
Really, all I am trying to say is don't let one class restrict you to a certain faction that you want to play. Your class will be available to both factions, just slightly different on each side. The differences should be subtle, especially since the mechanics are exactly the same. Also, since I was unable to find a list of it anywhere on the interent, here is a list of all the classes in Swtor and what their equal is on the opposite faction.