For backgrounds just experiment with filters and brushes. Thats all I did when I first got Photoshop. Just start with something simple like a cloud render, then change the colors (ctrl-u, click colorize), and then fool with the filters like the distortions and such.
Example: -Open a new document.
-Go to Filter>Render>Clouds.
-Hit ctrl-U and check colorize. Then make it any color.
-Go to Filter>Distortion>Glass.
-Mess with the options till you get something you like.
And you're done.
You should get decent-yet-simple looking backgrounds from fooling around like that.