I recommend picking up a few of SitePoint's design books--they're quite good and helped me pick it up quickly. Also, I'm building an HTML5 canvas-based mockup

I recommend picking up a few of SitePoint's design books--they're quite good and helped me pick it up quickly. Also, I'm building an HTML5 canvas-based mockup tool called jMockups, which I plan to eventually have export to quality HTML/CSS. The mockup tool will be ready in two to three weeks. If you're interested in helping test it, sign up for the mailing list on the jMockups.com homepage. Cheers.