There are many ways people can help out the Batman: Arkham Wiki, ranging from adding high-quality screenshots pertaining to the games to improving stub articles on the wiki.
For viewers who want to contribute, making an account is simple. Just go here and fill in the required spaces and there you go; you'll be an editor of the Batman: Arkham Wiki.
If you are unsure of what to do or how to create a page, search for a few articles on the same topic and see what they look like. You can always view the source code of a wiki and learn from what others have done.