This is what appears when you click "Change this page" at the top.
There are already 187,075 articles on this Wikipedia, so you might find that an article you wanted to create has already been made. That's okay, because you can add, change, and even take away information in articles that have already been made.
You need to click "Change this page" at the top, and the Wikitext of the page will show. Wikitext is the coding that allows pages to be made. You can change this to add your knowledge to the page.