Create an account! – This is optional, but it'll allow you to create a username for yourself and track your contributions, which makes it easier to follow-up on articles you've worked on.
Decide on a topic you want to work on. Use the lists below to help you decide. If this is your first time editing, consider working on an existing article rather than starting a new one.
For articles outside the list below, use the search tool to see if there's already an article on your topic.
Ask an experienced editor at the event if you're having trouble editing / referencing / finding sources / etc.
When you think your draft is ready, ask another editor to read it over and provide feedback. Be sure that your topic is notable and that it includes reliable sources like news articles, interviews in major publications, academic papers, and books.
Google Scholar - You can link this up with the Univ. of Chicago Library to get full text of publications through JSTOR and otherwise, and it's vastly more convenient to use than databases themselves.
Google News (newspapers) - A great way to directly check the text of articles in hundreds of U.S. and international newspaper articles.
Google Books - Google Books often allows you to read portions scans of books, so searching within a book for a topic can be a very good way to get coverage on your topic.