Just like in the previous Dragon Age games, your character can specialize in a selected area (while the rest of the party members receive it automatically), which increases his combat abilities (e.g. strength or constitution). It is highly probable that a specialization is initially unavailable and you can change that while you progress in the game. This means that you need to wait until a marking on the map appears (at the war room) which is connected with the option to play a quest of inviting trainers. Then, they appear at the Skyhold (to learn more, see the remaining guides for Dragon Age: Inquisition).
It does not suffice to identify the trainers, because they will be willing to share their knowledge with you only after you perform the tasks that they give you. Therefore, you need to meet the specific trainer, learn about the details of his task and complete it.
Note! It is a good idea to complete all of the challenges tasked with the trainers and, only then, decide which area you want to master.