There are two ways to look things up in Wikipedia: by searching or by browsing.

If you know the name of an article you are looking for, simply type it into Wikipedia's search box and press Go, or you can search for information on a topic by typing it in the box and selecting Search.

If you would like to look around the encyclopedia to see what is on it, use Wikipedia's Contents pages. Lists and indices are examples of contents for a published work, and Wikipedia has many of each, including a complete alphabetical index and indices by category.  Links to all of Wikipedia's main contents pages are presented below, and they in turn link to the more specific pages.