How do you unlock the dragon age origins specialization

some of your allies have specialization in an area
like Alistar's specialization is being a templar
to learn ones of your companions get their approval of you up and you may learn it from them.
Others may be found all over Ferelden like the Duelist
go to the pearl brothel and you see a woman fight, after she wins talk to her and ask to learn from her. she will play a game with you first and you use dexterity and see that she is cheating. then you unlock the duelist specialization.
google the locations of the others if you want them

and dont forget to select what you want to specialize in when you level up. its under the your character's picture.