I setup Drupal 8 in my localhost, i want to run multisite using one database and one instance (under one drupal installation) How do i achieve this?
First, read this page on Drupal 8 multisite, https://www.drupal.org/node/2297419.
It should give you a good idea of what you have to do in order to get multisite working and also explain some of the things you should consider. Following are generic steps, there are many things you can customize using settings.php.
The documentation inside the settings.php file is also very helpful, read that too by viewing the source of the file here https://api.drupal.org/api/drupal/sites!default!default.settings.php/8.
Briefly, the steps are,
In your settings.php there is the database connection setting. In there, you just need to provide table prefix that will distinguish tables for different sites.
Have a look at the settings.php file, it is well documented in there.