You've surely noticed the trash can sitting in the bottom-right corner of your screen. Looks like the kind of trash can in which you throw things away, doesn't it? It is.
Just drag any item you don't want (like one of those practice folders you made) over to the trash.
Make sure the tip of the pointer highlights the can, then let go and the item will drop into the trash. The icon changes to look like it's overstuffed.