Wikipedia is an encyclopedia, that anyone can add to. It has been written by people just like you, sharing knowledge with others. You're welcome to add to any article, or start a brand new one.
Most of the time, it just does. There are more people trying to improve Wikipedia than break it, meaning the encyclopedia becomes bigger and better every single day. Everytime someone changes a page, Wikipedia remembers what they changed - so if someone adds nonsense to a page, anyone can easily go back and fix it.
Start editing! For your first edit, try going to our sandbox page - where you can play about. Simply click the edit this page link, change the writing and click save.