You can write an article, for example a tutorial, correct spelling mistakes, add links to existing link lists and much more. Just make sure your contribution is related to free/open source game development.
If you want to contribute, but have no idea where to put it and no time to find out, you can submit to the stuff page.
Editing existing pages
Any existing article has the links "article", "discussion", "edit" and "history" at it's left top. These links are the keys to editing pages.
This is the default view, it shows the article in it's end-user form.
This page is used for talking about the article. If you want someone to change it or if you're unsure about weather your plans to change it are justified, write your request/question on the talk page.
Consider using the forum or IRC for discussions instead though.
(You will have to visit the talk page's edit or + link to do so.
This changes to the editing page for the current article or talk page.You can alter the content of the text box, which is the source text or markup text of the article and use the preview button to see how the changes you made would reflect on the article page.
On talk pages, there is a + link. If you visit it and submit text, it will be added to the bottom of the talk page.
If you want to take a look at the changes made to the current page in the past, visit the history link. You can compare the source text of different versions too.
Creating new pages
If you want to create a new page, enter the desired name in the search text field and press Go. Now press edit at the top of the page, to fill the article with content.
Important: Make sure new pages don't get lost! Categorize them!
To put an article into a category, add
at the end of the article. CATEGORYNAME is the name of the category, you can create new ones or choose from the existing categories.