Help:Introduction to editing with Wiki Markup/1
Editing pages
Activate editing mode
Formatting
Keeping it tidy
Wikilinks
Connect pages to each other
Saving your changes
Publishing
Creating new articles
Building a page from scratch
Summary
Review of what you've learned

Wikipedia is formatted using its own language called Wiki Markup. It's pretty simple to learn if you want to. Otherwise, you can use VisualEditor, a simpler editor interface that works like a word-processor]]
To view and edit a page's source code, just click "Edit source" at the top of any page. This will allow you to type in the text that you want to add, using wiki markup to format the text and add other elements like images and tables that are explained later in this tutorial.
Wiki Markup can initially look pretty intimidating (especially references) but it actually requires only a few rules to understand and use and you'll quickly get used to it! You can always test new techniques in your "sandbox" page.