Which distro uses lowest system resource (when installed on hard_disk/usb) where I can install software from Debian OR Ubuntu repository?
I found Quelitu which is based on Lubuntu, but uses less memory as given on their site. There are lighter distros but most of them are based on Slackware. I don't have much experience so want some expert opinion. Thank you in advance.
Bachata linux is perhaps the lightest I have found, but takes a little work to set up
The most practical light true-Debian distro I have encountered, so far, is antiX-M11 -- see here, then here (not a fair trial, IMHO), then here, to alleviate any concerns !
I use antiX on laptop, Salix 13.37 ( a Slackware-based distro) on desktop exclusively, now. Before that, it was peppermint for a long while.
Without much experience, you are wise to steer away from slack-based distros, for now. Suggest learning how to edit /etc/apt/sources.list file and becoming moe than just familiar with apt-get and aptitude in the command line. Continue learning how to edit other config files and know where they are located within the linux file system; then you will grow into being able to use a Slackware-based distro like Salix (which is quite easy compared to the Slackware of old).
The beauty of Linux: you can tailor it to your needs (some more easily than others).
I agree with Pyroflea, stock Debian, when installed without Gnome and instead replaced with a lighter DE/WM (like LXDE, I personally prefer Fluxbox, but to each his/her own) and you're looking at a system that, at idle after a boot, will be using 50-200 megs (Linux tends to cache as much as possible and keep stuff resident in memory until it needs to reclaim the space)
It depends on how much memory you computer has. If it has 500 MB RAM or more, I would definitely go for a Ubuntu derivative (Quelitu, Lubuntu, etc.) as they are more user-friendly and there is more software and a lot more information on the net for them.