For a general introduction to Wikipedia, see Using Wikipedia Wisely.
There are a number of ways to find articles to edit, or articles that need to be created in Wikipedia.
A WikiProject is a group of editors that work together as a team to improve Wikipedia, usually focused around a topic area. WikiProjects are a great way to find articles that need work or need to be created in a given field. Some projects may be inactive.
Search for projects here.
For example, Women in Red focuses on creating content regarding women's biographies, women's works, and women's issues (red links in Wikipedia indicate an article that needs to be created, hence the group's name). The Redlist index lists articles about women that need to be created by category.
Another example is WikiProject Philosophy. The Assessment section categorizes articles that need word by their quality grade (see "Content Assessment" page in this libguide). Pick an article and start editing!
Wikipedia's Task Center lists maintenance activities, organized by task, that editors can perform. To the right of each task it will tell you what suggested skill level an editor should have (ex. all editors, intermediate editors, advanced editors). Pick a task and start working on an article!
A more advanced version of this page can be found here.
Check Meetup for events, and the articles associated with them. You could join an event, or just edit articles that need improvement that the event identified.