The main idea to understanding photoshop is layers. Open a new canvas in photoshop and follow along. First, find your layers palette on the right side of you workspace. If you don't have a layers workspace, follow this tutorial.
Right now there is just one layer, the background. To make a new layer, click on the small icon next to the trash can.
Now a new, empty layer, called Layer 1, has shown up. Make sure this layer is highlighted by clicking on it once. Using the marquee tool (M) draw a rectangle, then using the paint bucket (G) fill it with a color different than your background. Now you will have something like this.
Now select the move tool (V). If the new layer is highlighted in the layers palette you will be able to move it. Now make a new layer with a new rectangle in a different color. Notice that when you a move a layer, only what is in that layer will move. This is where you get the power in photoshop. By putting each detail for your page on a different layer, you will always be able to move just one thing, instead of having to delete the whole thing and start over.