Coding a template

Hi,
I've made a template in Photoshop, and I've sliced it and everything, but I'm really stumped as to how to make the main content box expandable through coding. Any tips? I'd really really appreciate some help or a link to a good tutorial!
Thanks!

In how far are you familiar with (X)HTML and CSS?
What tool/editor do you plan to build this page/site with?
Could you give us a peek at the design?