I am thinking on installing Debian squeeze to replace my ubuntu 9.10 instead of going to 10.04 but I want to know how to add repositories among other things,can anyone please tell me how to step by step?
Not completely step-by-step, but this should be enough to get you started.
Firstly, the repo info is held in a few config files, namely /etc/apt/sources.list as a good starting point - take a look at the contents of that file for starters. It should be a plain-text file, so a graphical viewer/editor (or just the "more" command) ought to me enough to view it.
From what I understand, adding a new repo in is a matter of adding another line to this file (or a new file in the /etc/apt/sources.list.d/ directory) then running apt-get update to refresh the (locally-held) apt databases for it to be aware of the new software.
The process for adding a new repos for Debian *should* be the same for ubuntu - ubuntu is based upon Debian after all - but I'm guessing the HTTP/FTP locations will differ.
nb: ubuntu has an "aptitude" tool, as well as some graphical tools to manage apt repos. You may find the same tools under Debian graphical desktop, once installed.
Does that help?