VisualEditor is a What You See Is What You Get style editor for Wikipedia. It's pretty simple to learn if you want to. Otherwise, you can directly edit the source code using the WikiMarkup editor, the primary editing interface which works more like a plain text file.
This tutorial introduces the basics of using VisualEditor. Full instructions can be found in the VisualEditor user guide. VisualEditor is enabled by default for accounts created since September 2015, and can be enabled for earlier accounts through the Editing tab of Preferences.
VisualEditor is still in development and so has some limitations and bugs. Most notably, it is slower to load than editing the source code, and is available for article pages but not talk pages. If you spot any problems, you can leave comments and suggestions at the Feedback page.
To edit a page using VisualEditor, click on the "Edit" tab at the top of the page. (A long page may take a few seconds to load.)
You can also open VisualEditor by clicking on the "edit" link on each section of an article.