Don't worry if the prospect of editing Wikipedia seems overwhelming. You'll have a lot of help, both before the Edit-a-thon and during the session. Below, are some answers to questions you may have.
How do I decide what to do?
That depends on your interests. If you're coming to the Edit-a-thon for class, your professor will have some guidelines for you. If not, your options are wide open. Basically, you will either create a new page or make edits to existing ones. Editing an existing page is much easier than creating one from scratch. Existing pages will usually have a list of improvements that have been requested by members of the Wikipedia community. You can access these through the "Talk" tab. The Community Portal also lists several different ways to get started helping out. Many Wikipedia projects have "to-do" lists you can complete to support their efforts. There is a task list for Art+Feminism and several worklists for Women in Red. If you're willing to endure a little randomness, use Citation Hunt to find a single statement that needs a citation.
What if other editors don't like my improvements?
You may have your edits challenged or undone by more established Wikipedia editors. This is just a part of the process. Make sure to come back and visit your page later to see what additional edits have been made.
I don't know Wikipedia's mark-up language. How do I format my content?
Wikipedia now has a visual editor that makes creating content much easier. You can find VisualEditor here.
I want to write an article from scratch. What format should I use? How do I structure my article?
This guide is very helpful for creating your first article.