ToME: the Tales of Maj'Eyal

I am playing as a Berserker and want to learn the Superiority tree.

Do I need to pay someone to learn it?

Or is it something that needs to be unlocked?

It's a locked tree, so you "learn" it with a Category point (You'll get your first one at level 10; Cornacs (the race) start with one)

Once you have a Cat point, just click on the tree's name on the level-up screen