No problem, pomegranate! This is solved quite simply.

In Linux (as well as UNIX and BSDs) the important system settings and files are not allowed to be modified by the ordinary user, this is for security. The only way to make things work is to provide ownership to the system files not to user (you), but to some administrator, or, as we in Linux-world say, Superuser, also called "root".

If you go to console and try to look at the /etc/apt directory with
Code:
ls -Al /etc/apt
you'll see something like:
-rw-r--r-- 1 root root 5535 2006-07-30 02:41 sources.list
This means, that file "sources.list" is owned by user "root" and belongs to group "root". "-rw-r--r--" means that the owner (root) can Read and Write this file, the group (root) can only Read, and everybody else (this includes users like yourself) can only Read the file.

Luckily, there is a way to give a command to the system as if you were root. For example, if you enter
Code:
nano /etc/apt/sources.list
in console to edit the file, you do it as yourself and are not allowed to write what you have modified. But if you do
Code:
sudo nano /etc/apt/sources.list
this means that the command is issued by root, so you will be allowed to write the file. For security reasons, you will have to enter your password when doing something "sudo".