How do i add a eccomerce software on a website already made?

I already have a site made. I just want to add a shop to it. I dont get how to make a site work with something like virtuemart? How do i do something like this?

 

Virtuemart is a Joomla module. Are you using Joomla?
I'm not sure how active the Joomla community is these days. Drupal might be better. Or Wordpress. Drupal would support a large, multi-functional website better than Wordpress, imo.

Here are the most important questions to answer before committing to any ecommerce scripts, modules, or services.
Is the "cost" of setting it up and running the particular version worth it? --Meaning if you pay a service to maintain/host the store, but you're getting great ROE, then maybe a paid service is worth it.
Is the script easy to use? --No script is worth it if all your time is spent trying to use it.
Does it provide you with the necessary data to run your business? --No reports? may not be worth it.
Is the script still being updated and maintained? --This is really important, as you don't want to commit to entering all those products only to find that it won't work with the latest browser versions or php version.
Is there a large and active community? --Also very important. You would want an active, supportive community to be able to help you when need it. Also, a good community will be there to provide modifications or plugins for it.
Is the ecommerce script modular and built on a modular platform? --Unless you are a php guru you should avoid scripts that are not.

 

There are many free open source shopping cart software available and i suggest zeuscart which is the best advanced open source shopping cart software to create your store.