Depth I can understand, because that is very hard to pull off sometimes.
It's basically how you use blur and sharpen layers, obviously mixed with other elements..
composition ties into it, especially when dealing with focal perspective.
Composition is easier though, and you should really work on this first..
because honestly I'd rather look at something that lacks depth,opposed to a completely sloppy canvas.
All compo is really is how you place everything. You obviously don't want your focal overcrowded,
and you don't want to just throw effects all over the place in random areas.
Sometimes it can work in abstract pieces, but that's tough to pull off.
When dealing with a single focal, you really want that to be the focus..not a mess of effects surrounding it.
I'm really considering team viewing you just to watch you make a tag, would be easier to help you that way.