Then go pick out well designed websites and try to recreate it with Photoshop. You can use Dribbble for this. Try to pay attention to every pixel and recreate it exactly as you see it, or it'll most likely end up ugly. After a while you can try to add your own stuff onto it.
After doing this for some time, you'll have learned how to do simple stuff with Photoshop, you'll have learned what kind of layouts each type of website uses and you'll be able to use these layouts for your own projects. You'll learn how to effectively apply subtle effects and much more.
Basically, the fastest way to become good at design is to copy other people's design.
Also, remember that anything you design at first will probably look ugly, but you need to keep tweaking it until you find something that looks good.