Before the reader is captivated by your words they must be drawn in by the "curb appeal" of your cover. Yes, they judge a book by it cover.
If you are lucky enough to have a professional graphic designer to help you and you have the money to pay them, this article probably won't interest you. Those of you on a budget, read on.
Color: Black or red backgrounds suggest danger, fear, guilt, death, or anger. White or blue-colors makes you think of something soothing and peaceful. Select your genre and check out other successful books to see what they used. I have even heard of a book that used the same graphic in four different background colors.
Text: Just be reasonable. We all want to be different, to stand out, but please don't use unreadable fonts! Rule of thumb is usually title on top, your name on the bottom. Nothing is written in stone. Try to use a color for the font that compliments the major background cover. Above all, remember it must be fairly readable in a thumbnail view. Most of the time that is how it will be viewed initially.
Artwork: Photos or drawings, it's up to you. Make sure the graphic reflects what the book is about without being too busy. Too many objects will only confuse people. Color is as important here as the background. You have three elements that must mesh together in such a way that it looks as though they are one. Whatever you use it must be original art, free to use, or you must purchase it and give credit, if necessary. Be very aware of copyright infringement when choosing art from the internet, read carefully all the rules for where you acquire it.
Putting it all together: Paint does an okay job. I have Photoshop, which I love, but I had to buy it. Good news! There is a free program called PAINT.net that is made to mimic Photoshop. You can download it here: http://www.getpaint.net/ Small learning curve and you will be making great covers!
This is a "Get Started Quick" Guide. There are other programs to use, everyone has a favorite. These are the best choices in my opinion. Most of the advice here is common sense, but I believe using these simple tips, you can come up with something beautiful.