When working with PL/SQL procedures, you'll have to comment (and uncomment) frequently part of the code.
A quick way to do it with Oracle SQL Developer is selecting the block of code you want to comment and use the shortcut:
Ctrl + '/'(slash)
Alternatively, you can choose the option 'Toggle Line Comments' from the menu 'Source' (in version 4 and above of SQL Developer). In previous versions, use the menu 'Edit -> Source'.