In the beginning you have only your primary profession in any campaign. Almost immediately though the storyline/quests let you begin to experiment with the other professions as a secondary. I always recommend that you try them all out before you make your final choice, if nothing else you can learn some free skills.
You will come to a point where you
must choose a secondary profession that you will carry with you for a while.
Eventually each campaign also takes you to a point where you are allowed to alter your secondary professions at will in outposts.
http://gw.gamewikis.org/wiki/Main_Page
Great place for just raw information about GW but sometimes hard to navigate when you are looking for something specific. Great resource though, highly recommend it.
In the beginning I personnaly would ignore your secondary profession as a warrior because you will have a hard time with both the energy requirements and the attribute requirements needed, especially early on. Many great warrior bars have only warrior skills on them.