But how do you know what elements to use and where do you find them? Simple: collect them. Say you want to draw a forest. What kinds of things would their be? Mushrooms, trees, grass, plants... what sorts of animals? You can look at real forests if there are some near you... or search for photos online. Field Guides to animals (birds, butterflies, snakes etc.) and plants (trees, mushrooms etc.) can come in very handy for ideas. I often make sketches of elements in my sketchbooks so I can use them later. One example of a page of collected elements might be:

The elements above are sketched in pencil in a fair amount of detail, but don't worry - it's possible to make backgrounds out of even a few very quick, simple elements. to begin with you might try these:

Next you might try to come up with at least two ways of drawing each element so that your pictures have variety. Keep collecting them and pretty soon you'll be able to draw five kinds of mushrooms, several types of plants, and maybe a number of styles of rocks. While you make have started by looking at the "real" thing, after a little practice you can begin to create your own elements by combining different elements you've learned.

I hope this quick basic look at backgrounds has been helpful, please stay tuned as I plan to add more to this set of tutorials pretty much indefinitely!