There are two ways to manage multiple domains in one Typo3 installation. The first method is to create a RewriteRule in your .htaccess which tells the webserver to redirect requests on a specific domain to a specific page id / website tree. You can do that by putting the follwing code in your .htaccess

RewriteEngine on

RewriteCond %{HTTP_HOST} ^(www\.)?testdomain\.it$ [NC]

RewriteRule ^$ www.maindomain.it/index.php [R=301,L]

RewriteRule ^[^/]*\.html$ /index.php

The second and much better way to handle multiple domains in one Typo3 installation is to use Typo3 Domain Records. You create your page trees the usual way. Afterwards you create a domain record on each subtree for each domain. This way can you tell Typo3 which subtree is called on which domain.

Requirement to get this working is that DNS and Webserver are configured correctly for your Domains.

More information can be found on:

Quickstart - Controlling multiple domains

Using subdomains with Typo3