I don't make wallpapers, but I do make sigs, and the process is pretty much the same
First you have to get a picure editing program. I use Photoshop, and I'd probably recommend it to beginners--it's fairly self-explanatory, and all the little effects things let you turn out some pretty cool stuff once you get the hang of it. However, there are several other programs out there--namely Paintshop Pro. Both of those can run pretty high, price-wise, so if you can find a pirated version online, it's almost worth it...
Then, you gotta go find some pics. Start simple, don't try anything too extravagent at first. Just make a background and arrange a couple pictures in a good looking way. Then, you'll probably want to add some text... That's easy though...
Also, I find it useful to look at other people's work for ideas. When I first started making sigs, I really had no idea what I was doing, so I copied Godshinto's work. It was all fairly simple looking but still "pretty". 馃槉
Have fun and May the Force Be With You. 馃檪 馃檪