How to install a software

I have been talking about alternatives of windows software for linux and other softwares, but the very question that how to install it , remains unanswered , so I am here to answer that.
First of all you should know the concept of repositories because in linux softwares are not installed by downloading a single .exe file.
In linux a repository is a server where all the softwares are present pertaining to a particular distro lets say for ubuntu 9.10.
To install a software you need to install it through synaptic package manager shown in image

Open that and you will see a list of softwares that are installed onto your machine but can also show softwares that are not installed (only if you have added repositories)

If havent added any repository here is a procedure to add one:

-Open a terminal (command line mode)

-Type in sudo gedit /etc/apt/sources.list

• Add the following lines to it :- ( only for IIT Delhi as it has its own local repository but for others a simple google search for Ubuntu 9.10 repositories will yield you a list like this for you)

deb http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic main restricted universe multiverse
deb-src http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic main restricted universe multiverse
 
deb http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-proposed main restricted universe multiverse
deb-src http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-proposed main restricted universe multiverse
 
deb http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-updates main restricted universe multiverse
deb-src http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-updates main restricted universe multiverse
 
deb http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-security main restricted universe multiverse
deb-src http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-security main restricted universe multiverse
 
deb http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-backports main restricted universe multiverse
deb-src http://poorvi.cse.iitd.ernet.in:80/ubuntu karmic-backports main restricted universe multiverse

After adding the repositories click the reload button as shown in synaptic manager so that the list of softwares in the repo (short form of repository) can be listed in your package manager

After this you will get a list( I havent added any repo so you can see all softwares icons are green means they are the ones already installed on my pc) and just right click on the software you want to install,then "mark for installation" and then click apply in the toolbar in package manager 

You are done your software will be downloaded and installed by its own.