Time and practice are the best teachers. If you can tell what (you think) looks good, then you have already solved half of your problem. Spend a few hours tweaking and improving on a design until you are able to come up with a design that you like. Look for inspiration on movies, photographs, nature or other websites.

I remember spending a couple of days on my old website design. I listened a lot to soma.fm and I noticed that the music kept me focused on the design I was working on. I remember dreaming about what I am going to tweak/change the next time I opened Photoshop.

Also, learning to imagine and think ahead on "how to do this and that in HTML/CSS" will save you a lot of time and effort.