Active Skill System, Item Quick Selection

Hi guys, I'm looking for something really more complex that anything I've done before (well, I haven't set the standard really high though ^^)

I already have a lock/unlock Skill System (basically, an array with skill ID -> and 0 (lock) or 1 (unlock) value for each skill. If the value for the iD is 1, the skill is unlock. Now what I'd like is also to have a "quick selection" skill menu : the classic action rpg like, a square item that shows the active skill, when you press the "up input" (on a controller let's say) the item displayed is the next skill unlock, and obviously, when you trigger the "skill button" the active skill is fired.

Now I gotta say, I don't really know how to manage all this, I know a very few about GUI (which reminds me I've got another problem, if you guys take a look here : http://answers.unity3d.com/questions/1070397/set-public-image-in-canvas-from-script.html ) Any help or inputs would be really appreciated !