I've found some website that is very concise in the usage of CSS in the website, hope these websites could help you to finish the homework.
http://freewebs.com/csskit/ (a lot of samples in CSS, properties)
http://brainstormsandraves.com/articles/semantics/structure/ (this one is quite beautiful, also it has some information in XHTML and how to write it, suitable for some classmates who is not very familiar with XHTML)
I am still investigating the usage of XHTML and CSS as this is my first time to use such stuff to write a page.. But I would try my best...