Hi @Andrea_Manca and welcome to the forum 
To get started, here are some resources:
The manuals are here: https://docs.greenbone.net/#user_documentation and probably the best place to look at is https://docs.greenbone.net/GSM-Manual/gos-21.04/en/ which should give a good view on how things work.
There are also training classes online so take a look at GSM for Starters, and info about that is in this thread First Self-Learning Course "GSM for Starters"
Hopefully that helps to get you started and please let us know how it goes. If you have any specific issues as you go along please let us know about your setup and we’ll try to help.