This will come naturally as you play the game so try not to worry too much about it.
Dismantling items is accomplished by opening your inventory (Options). From here you can see what you have equipped, as well as any other items you have collected along the way.
To dismantle an item, simply hold your cursor overtop of it and hold down Square. After a second or so the item will be dismantled and you will awarded Glimmer (as well as other materials when you break down anything higher than white gear).
I personally got this on my first playthrough around Level 9 but I'm sure it will be different for everyone.