[Mujica]

You have to buy the skills as they become available to progress to the stronger ones like in most games. Leveling up skills however is a little more complicated than that as you can decrease its SP Cost, increase the damage it causes, increase its DOT ( Dmg over time ) timer, or decrease it's cooldown. Choosing which to put into which skill is all up to the user and certain combo's of different attributes into ea of your many skills can give a player a really good advantage or none what so ever, it depends entirly how you use your skills and how you base your points around that.

EDIT:
To level up skills simply open the upgrade box VIA the button, left click the skill in the skill menu once, drop it in the upgrade box, and proceed to press the arrow in whichever catagory you wanted to upgrade it in. Choices ar eclearly stated next to each upgrade panel.