Wikipedia has two editing interfaces. The easiest to learn is VisualEditor, which does not require the use of markup language. The other is wiki markup (wikitext). Tutorials for both are found here. Additional tutorials are below.
Editing Wikipedia: A Guide to Improving Content on the Online Encyclopedia
Excellent starting point to all things Wikipedia.
A self-paced orientation tutorial for students editing Wikipedia as a class assignment.
Wikipedia How-to guide: Editing
A basic how-to guide for editing Wikipedia.
In Wikipedia, article content should be verifiable through reliable sources. When editing or adding text, you can document your sources by adding citations which will appear as footnotes in a References section at the bottom of the article. If an article does not have a Reference section, you can add one.
How to add a Reference section:
How to add a citation: