Drupal 8 - how to run multisite using one database
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,


Multi-site requires the existence of the sites.php file in the sites directory. This file does not exist by default.
Add your sites to a $sites array in this file, instructions are mentioned here https://www.drupal.org/node/2297419
Create a site directory inside sites/, the path should be sites/site1.
Each new site directory must have a settings.php file in order for Drupal to recognize it.
Visit the URL of site1, site1.yourdomain.com to install it.