In Visual Studio? Just select the lines you want to comment out and
press the button 'Comment out the selected lines' on the Text Editor
toolbar. If that toolbar isn't visible, you can change that via the
View/Toolbars menu. You'll see that it also has a button to uncomment.
If you prefer shortcut keys: Ctrl+E followed by C (comment) and Ctrl+E
followed by U (uncomment).