Multisite for dummies

Set up Multisite in the normal way, then add the Domain Mapping Plugin and use that to map any additional domains to individual sites in the network.
https://wpmututorials.com/plugins/donnchas-domain-mapping-plugin/

here’s how to set up multisite:

https://codex.www.ads-software.com/Create_A_Network
https://wpebooks.com/2010/09/how-to-enable-multisite-in-wordpress/

thank you so much!! I will try to do it now

Another question. I already have one website working using WP. If I add the multisite and the domain mapping plugin, my website will be erased?

No.