the best way to get root permission is type:
in a terminal (console).
then your root password.
you can use vi or a similar program to edit the file. (you can also start a gui program by typing the command as well)
if you don't understand this you need to search for some basics tutorial. (www.linux.org
has one). the commands (especially in vi) can be non-intuitive at first.
... uh oh, it's homework?