Making changes to encyclopedia entries can be a little intimidating, so Wikipedia provides editors with a scratch pad to try things out first. These pages are called "sandboxes" and there's more than one available to you.
Assuming you've created an account, you have your very own sandbox found via link at the top of any page. Wikipedia also maintains a communal sandbox for anyone to test edits. You can copy wiki markup code from any page and experiment with changes in the main sandbox or your personal one. And then you can erase or delete it all once you're done.
Keep in mind, however, that even on sandbox pages, the same rules apply: respect for copyrights and for other editors are just as important here as anywhere. Also worth noting, just because you can blank the slate, past sandbox edits are still accessible to others.