I tried to follow the link bellow to configure the multisite.
https://learn.getgrav.org/advanced/multisite-setup
Someone can help me please ?
I’m totally lost 
For brevity I will link you to https://getgrav.org/forum#!/general:multisite-inheritance-quest . There should be all details you have to know about multisite setups. Further read the linked references therein.
I tried but … nothing worked
Ok, I really don’t know what went wrong in your case. Did you put the setup.php into the Grav root folder. Did you point all subdomains to that folder, too? Can you attach a screenshot of how your directory structure looks like?
I’ll do it soon, once i tried again. thank you for your help.
what is for you the grav root folder ? for me its the www folder where I have the different folder (assets, user, system, etc… )
That is the correct location for setup.php, what does your VirtualHost setup look like? This varies by server type and OS, but for example using WAMPServer which uses Apache your folder setup will work.
As @Gingah said that is the correct location. In order to make multisite to work you of course have to setup your webserver correctly.