VirtueMart is a free eCommerce component for Joomla. This section contains some tutorials on Virtuemart.
You can find Frequently Asked Questions linked here. There is a knowledgebase here, an user manual for end-users, and Developer Manual.
If you need to ask question, there is the VirtueMart forum.
Note that the VirtueMart component (com_virtuemart_xxx.zip) is over 2 MB in size. Some PHP environments prevent the upload of such large files via the web admin. So if you have problems installing the VirtueMart component via the Joomla Administrator's "Extensions -> Install" web page, try unziping the com_virtuemart_xxx.zip and uploading it to your Joomla /tmp folder. Then point to that folder using the "install from directory" option in "Extensions -> Install" menu of the Joomla Administrator.
If your installation is successful, but you are getting Javascript errors when you are using the VirtueMart admin, perhap the forum thread linked here may help.
