Discover Color Fill
Make a selection of the simplest area to select (in this case the sky) with the magic wand tool. We are soon going to cut the sky out of the background and put a new background behind the cactus.
Duplicate the original background layer (as I always teach) by dragging it to the new layer icon as shown. You want to keep an original of the image so you can come back to (in case of emergency) for G.P.
Here I've dragged (drug, drugged?) another copy to the new layer icon to retain an original and to fill this layer with a color in the next step.
You can also just click on the new layer icon itself but remember
that it will create a new blank layer above whatever layer is
currently selected in the layers palette so you will have to select
the background layer first (or you can move the layer order
Edit: Fill is one way you can fill. Take note of what layer you want to fill and make sure that is the one that is selected in the layers palette.
Now select the layer you want to cut the background out of ( in the layers palette).
You may have to re-select the background with the magic wand tool (in this case). Edit: Cut to get rid of the selected area (the blue sky). Note how the cactus is left on its own.
You have cut or erased the pixels you selected on that layer. This is basic stuff you must understand before you can move on to really cool stuff, oh apprentice.
The paint bucket is another way you can 'fill' a layer with a color.
Choose a color from the color or swatches palette (the eyedropper will automatically appear).
This will now be the selected color that appears in the foreground color chooser in the bottom of the toolbar. Make sure you are on the proper layer in the layers palette that you are going to fill and get rid of anything else that is there.
Choose the paint bucket and click it to fill the layer with the color. You can also use Alt/Opt backspace to fill with the foreground color.
You can also choose pattern fill from the options bar or from the Edit: Fill menu.
Just click and the entire layer (or selection) will fill with the pattern you've chosen.