"Where do I begin?"
Wikipedia is an encyclopedia, and contains articles which explain over a million different things about the world, the people and things on it, and beyond. There are two ways to look things up in Wikipedia: by searching or by browsing...
If you know the name of the topic you are looking for, simply type it into Wikipedia's search box and press Go.
If you would like to look around Wikipedia to see what's on it, keep in mind that every major subject has a root article on the List of overview articles and a portal on the List of portals.
There are several other "Tables of Contents" type pages on Wikipedia. They are the:
Wikipedia also has an alphabetical index of all of its articles, and a categorical index of most of them.
Read more: Wikipedia:Contents