If you want to do your own work and don't want to spend a lot of money on a graphics package, may I recommend Paint.net. My son did the cover seen on my avatar, using PhotoShop. I did the cover for
SitRep Negative myself. I enjoy fooling around with graphics, but nothing takes the place of artistic talent. Still, I think you can teach yourself to turn out some pretty decent work. As always, the simplest approach is to look at other covers, find something that appeals to you, and then make it your own.