In linux there are certain areas and processes that require root privileges to change them. To gain temporary root privileges type sudo <your command> if making a change in a terminal. If the program is gui based then from a command prompt it would be ksudo and the command. So in your case, you could ksudo <your favorite editor> /etc/apt/sources.list, hit enter, and type in your password. Edit the file and save it. You are done. Or you could edit a copy, save it to your home directory, change the permissions to match the original file, change owner ship to root, then sudo cp the file into the /etc/apt directory. I will leave it up to you to research chmod(for changing permissions) and chown ( for changing ownership ).
I am sure there is probably a better way but I usually edit the file with vi in a terminal. Being new to linux you would probably want to steer clear of vi for now.