You can unlock skill slots as you upgrade your Characters.

You can obtain or exchange skills with [Skill Ticket] or [Gold].

The Skills that you can obtain differ depending on the Friends character, 

so please check the [Skills List] on the right side of the Skills window.

In addition, one character cannot have more than one identical skill, so please take note of this as well.

* Actual in-game footage may differ from the screenshot.