Here is a link to a site I really like: http://www.tutorphil.com
I found this site when looking for rules about using contractions. It also has an entire list of links for other topics about writing essays such as thesis sentences, introduction, grammar, etc. You have to do a little navigating around to find exactly what you want but it is a self-tutorial of sorts, complete with explanatory videos. Topics include:
Essay Writing Tips
How to Get Started
Getting It Done
Introductions and Conclusions
TutorPhil.com is subtitled “College Writing Skills for Adult Learners,” but the site is helpful for high schoolers too. He does a good job of breaking down the concept of writing an essay in steps that are easy to understand.