With the exception of a few protected pages, every page has a link that says "edit this page", which lets you edit the page you are looking at. It is Wikipedia's most basic feature, and allows you to make corrections and add facts to articles. If you add information to a page, please provide references, because unreferenced facts can be removed.
Go to the sandbox and click the "edit this page" link. This will open an editing window containing the text for that page. Add something fun and interesting or "Hello world!", then click Save page and see what you have done! Be careful to make sure that you are editing the sandbox page, and not the text of this tutorial.
One important feature to start using now is the Show preview button. Try making an edit in the sandbox, then clicking the Preview button instead of Save Page. This allows you to see what the page will look like after your edit, before you actually save. We all make mistakes; this feature lets you catch them. Using Show Preview before saving also lets you try format changes and other edits without cluttering up the page history. Do not forget to save your edits after previewing, though!
Before you hit Save, it is considered good etiquette (or "Wikiquette") to enter a polite explanation of your changes in the Edit summary box between the edit window and the Save and Preview buttons. It can be quite short; for example if you just enter "typo", people will know you made a spelling correction. Also, if the change you have made to the page is minor, such as correcting a spelling or grammar error, be sure to check the box "This is a minor edit" (this is only available if you have logged in).