Hi Alessandro - To add to what Giovanni has said, it's unrealistic to expect to learn 'everything about Moodle'. You haven't mentioned if you're looking to use Moodle as a teacher, as an administrator, or as a developer? Each of these is a big field. There are training courses available, as Mary has pointed out, and there's a lot of documentation too.
Cheers,
Ravi
Cheers,
Ravi