A Wiki is a special type of web site that lets you and other visitors edit and add to the pages you find here.
As you learn, you can practice on the sandbox or playground, which is an area set aside just for testing and learning. Feel free to add and/or delete things there, but don't leave anything important there. That's just temporary space and the next person learning in the sandbox or playground may erase the stuff you did, or may simply add to it. If you want something to last, put it somewhere else in the existing Wiki pages, or create a brand new page for it.
Once you get the hang of editing Wiki pages, you should learn HowToLogIn, and create a listing for yourself on the Users page. Then you can set up your own page and let others learn more about you and your interests, post links to your website, projects, etc. See xiffy for an example.
Dive in, and have fun! Welcome to the Wiki Community!