I'm presently focusing on a Joomla website getting ecommerce functionality. I'm a beginner in joomla. I have to modify my virtuemart theme based on my very own css and html. How is it feasible?

Simply mind to your_joomla_directory/components/com_virtuemart/styles/your_theme_folder/theme.css

theme.css edits virutemart elements only, you may even need to edit your primary template css file to obtain the full preferred effect.