You need to configure your virtual hosts. In directory /etc/apache2/sites-available/ you may find example of virtual host files. So the steps are:
1. Create a virtual host file
sudo nano /etc/apache2/sites-available/site.com.conf
and set a content
<VirtualHost *:80>
ServerAdmin admin@site.com
ServerName site.com
ServerAlias www.site.com
DocumentRoot /var/www/site1/html
</VirtualHost>
It is a simple example. You can read manuals, to get more. Now just take in account, that you need to configure separate file for each site you want to run.
2. Enable your VH
When you finished your VH config, you need to enable your host by command:
sudo a2ensite site.com.conf
3. Reload Apache
To apply new host, you need to reload Apache
sudo service apache2 reload
Now it should work.