To play with layers, try this:
-- open up any photo you like
-- look for the floating Layers window (or choose Window from your menu bar and look for Layers--click on that to make the Layers window open)
-- in that window there should be a tiny thumbnail of your photo and the word "Background" in italics beside it. Right now there are no layers so what you need to do is double-click where it says "Background". Doing that opens up a New Layer box: by default it will be called Layer 0 but you can name that layer anything (e.g. "my test photo") and when you click okay, your image will now be on its very own layer!
-- now to experiment...another trick in Photoshop is "click on the little triangle", meaning that every window including the Layers one has a tiny triangle you can click on in the top right corner of it. So, click on the tiny triangle at the top right of the Layers window and a whole new world of options will open. You can duplicate your newly created layer or create a brand new blank layer, for starters. Let's choose to create a new, blank layer.
-- Then, pick a foreground colour, any colour. Choose Edit-Fill and click on Foreground colour to make your colour fill that layer. Your photo will disappear but not for long.
-- The last part of this layers tip/tutorial is that every layer can be as transparent as you want and the layer can also interact with the layer below it. To change these, you again need to look at the Layers window. The Opacity box that you see can be clicked on and when you slide it down from 100%, your photo will slowly reappear with the colour you selected layered on top. To the left of the Opacity box is a "Normal" box (that lets you change the layer type): when you click on that, other options will let you have the layers interact with each other in different ways, from dodging and burning (traditional photography techniques) to screening and overlaying.
It is really impossible to cover all of the facets of layers in one tip but, trust me, they are worth learning & experimenting with! Have fun!!