apt-setup is an interactive program that simplifies adding sources to apt's sources.list. It knows about all the major debian mirrors and can help you select one. It can even use apt-cdrom to scan CDs.
If you want to run apt-setup use the following command
You should see the following screen from here you can select archive access method for apt i have selected http and select ok and press enter[thumb:313:c:l=p]
Now you need to select the mirror country and i have selected uk and select ok and press enter[thumb:312:c:l=p]
Here you need to choose the debian mirror from the available list and i have selected www.mirror.ac.uk select ok and press enter
Now you need to enter the proxy information if have one otherwise leave blank select ok and press enter
This screen you can see it will test apt sources and it add these source to /etc/apt/sources.list file[thumb:316:c:l=p]
it will ask if you want to add another source and i have selected no[thumb:317:c:l=p]
That's it this is very useful if you want to add more apt sources for your debian system.