Do you have advice for a first-time webpage builder?

I foolishly volunteered to build a webpage for the lab I work in. I built one for my last lab using Dreamweaver and a template provided by the department. This time around, I have to start from scratch.

I am planning to use Komodo to write it in XHTML with CSS. (The internet tells me this is the correct way to do these things.) Do you guys have any general advice regarding either programming or design? The closest I have ever come to designing anything is painting one wall in my bedroom and I don’t really know where to start with this.

What are some common mistakes beginners make, and how can I avoid them? Are there any tutorials you would recommend, again on either programming or design? So far I have been using w3schools.com.

Thanks!