blue777 do I need to remove the current joomla site my client created?
Quickstart install joomla core plus template and extensions exactly like you can see on or demo site ,so you should remove the actual joomla installation and replace it with quickstart file
blue777 so how can I create database?
If you can use phpmyadmin then it is quite simple to create a database.
blue777 and which folder should i place the quickstart file?
It depends on how your client need website , if he wants it installed on the main domain .www.yourwebsite.xxx then you should copy quickstart files in main root of the server -
Hope it helps.