The sources.list file is located at

/etc/apt/sources.list

Please make sure that the version name (i.e wheezy) is used rather than stable otherwise it will cause issues when the next version of Debian/ubuntu is released - normally one of the benefits of a Debian based distribution is the ability to flawlessly upgrade to the next version when it is released however if you have Plesk installed that is not possible.

(if you are using a legacy       Virtuozzo and have a blank (or no) sources.list file please email [email protected] and we can add one for you)

Before you search/install a package please refresh the package list using this command -  

apt-get update

i ) To search for a package :-

apt-get update

apt-cache search [package]  -

i.e :-

apt-get update

apt-cache search MySQL

ii) To install a package :-

apt-get update

apt-get install [package]

i.e :-

apt-get update

apt-get install postgresql-8.1 

iii) To update your system

apt-get update

apt-get upgrade

iv) Install an .DEB file.

You can install single/multiple .deb packages by using the syntax

dpkg -i bash.deb

Please note that due to dependency issues it is better to use apt-get to install packages.