Presuming you only want one cover then it is probably far easier to pay someone to do it for you. A good cover designer will listen carefully to your requirements and allow you to make suggestions so that you end with the cover that you want - saving your hours of frustration with software you're not familiar with.
Covers can be great fun and if you keep it simple, not too hard at all.
Your best bet is to find a strong image that represents your book and add your title and name to it. Google image search even has a feature that will filter image search results by license (it's under their advanced options).
You'll want something either in the public domain (like art work or images taken and released by the govt - NASA has some great ones) or something with a license that makes it okay for commercial use.
After that, remember that you'll also need a font that is licensed free for commercial use. Thankfully, a Google search for this will also net you some fine options as many font places like to throw a few out for free in order to build relationships with potential paying customers. Remember: free doesn't just mean free to download, but free to use commercially as well.
If you want something specific, don't overlook the idea of hiring an artist to draw you something. Over on deviantart.org you'll find a wide variety of styles/media on offer and many are at reasonable rates from talented people just looking to make some money in their spare time. If you go this route, be sure to ask for references and specify this is a work for hire and the right to commercial use is part of the cost. They might even be willing to do add the Title/Author info. (If you find someone with a good-sized fan base, they may even buy a copy to show support for the artist.)
Good luck, and remember simpler is always better (especially when you picture how it will look in thumbnail size).