There are lots of great articles about Wikipedia and how you can contribute to it right on the website:
Starting an article (start here!)
Article wizard: Helps you through the process of submitting a new article to Wikipedia.
Notability guideline: Test used by editors to decide whether or not a given topic warrants its own article.
Manual of Style/Lead section: Refers to the section before the table of contents and the first heading.
Wikipedia Cheatsheet: A quick cheatsheet with Wikipedia mark-up editing commands.