You'll get this automatically as the story progresses.
Basically, you get a weapon called Hawke's Key and you have to upgrade it. Every floor level you get to a room with an alter type thing and once you activate it, you fight a demon then have the ability to upgrade it. There are three floor levels, meaning you get three permanent upgrades. You get four choices on each floor level.
The first floor gives the options of:
The second floor offers:
And the third floor offers: