It's been awhile but i believe the templar is unlocked from the beginning of the game you
just need to be a warrior to learn it.

If not then you need to make alistair like you to where he'll agree to teach you how to
become a templar. But it will not make you one. You need to select it from the level up
menu, and in order to be able to choose it you need to be a warrior.

I was wrong about one thing, you can buy the Templar specialization book from the dwarf
merchant in your party camp. This will allow you to learn the templar specialty, but for
warriors only.

I would have to assume that if Alistair teaches you the specialty you won't have to buy
the book. However, as stated above, you won't become one on the spot you'll have to select
the templar specialty from the level up menu, and only warriors can learn it.