First of all you need to download VirtueMart. You can download all available version from the VirtueMart Developer Portal - dev.virtuemart.net.
The official Download Page is http://virtuemart.net/downloads.
VirtueMart is available in different distribution formats.
Important
The "Complete Package" and the "Manual Installation Package" are available in two basic versions: one for Joomla! 1.0/Mambo and one for Joomla! 1.5. The second one has the extension ".j15.zip" (or ".j15.tar.gz").
This Package is a modified version of the official Joomla! Distribution. It already contains a preinstalled, pre-configured VirtueMart, so you don't need to install it separately.
As this bundle is a complete Joomla! package, you can't use it for an existing Joomla! installation, but only for a completely new site.
The complete package is an archive with all the things you need to install VirtueMart on Joomla! or Mambo using their automatic installer system. You can extract the parts from the complete package and upload&install them one by one.
The Manual Installation Package contains all the files from VirtueMart in the typical Joomla! directory structure. It doesn't contain any installable items.
This archive is meant to be uploaded into an existing Joomla!/Mambo installation via FTP. After that VirtueMart can be accessed and the necessary table structure will be set up.
If you want to upgrade VirtueMart 1.0.x to VirtueMart 1.1.x, this is your choice.