Layout
Brainstorm some ideas on paper. I prefer to start just in black and white. Colors can be distracting when trying to create the “perfect” layout.
Then use a grid system like 960 Grid System or Blueprint CSS . Some call them CSS frameworks.
Colors
Use a color generator e.g. colorschemedesigner.com and kuler.adobe.com and then pick colors from the theme.
Typography
A practical guide to web typography is: webtypography.net
Or use something like typography.css from Blueprint CSS