First we need to make a background "layer". Click on Layer, then click on Duplicate Layer and you will see two layers on your Layer Screen.
Since we do not want to have an image in our background layer, we will need to click into that layer on the layer screen so that it is highlighted dark blue. Now click on Select, then click on All.
Click on Edit, then Click on Clear and the background layer will be cleared.
Now click on the color you want in your Color Screen (if your Color Screen is not showing, click on Window, then click on Show Swatches.) I have chosen green - which is now the color square on the Tool Bar. Click on your Background in the Layer Screen to highlight it. Click on the Paint Bucket on your Tool Bar. Click anywhere in the Background Layer and your background will be filled in with the color you have chosen.
If you want to change that color, just repeat the steps using any new color.