Standard Permissions for /var/www after Fresh Linux / Ubuntu Installation with LAMP

Standard Permissions for /var/www after Fresh Linux / Ubuntu Installation
with LAMP

I just installed LAMP Stack with PHP-FPM & FastCGI. I have two users, root
and herit (with sudo rights, i can create more non-admin users if
required)
Now what should be default permission settings for my situation? I will
configure VirtualHosts for my domains inside www. Let's Assume i have
three domains, super.com, fruit.com & apple.com. I want separate user for
these domains (caz i have heard it is secure), so we anticipate fruit.com
& apple.com's script are vulnerable, i don't want them to mess up with any
other domain, because if any shell script or malware gets injected then i
am ruined. while super.com is having a secure script so i do want
super.com's user to have possible access to other domains, fruit and
apple. because my script on super.com is a web based file manager so i can
use it to control my other domain's data. i might have upto 10-20 sites,
each site can control themselves totally, while super.com which controls
everyone!
Additional Question: Do i really need 775 over 755?
All I am just worried is, that i don't want to get my site hacked as being
a newbie & someone overwriting my homepage, "You failed at setting
permissions!"
i am totally new with linux so i don't know much about chowning, so it
would be more appreciated if anyone can send solution with complete
command (i mean not just verbally described idea) so i simply have to
execute them over ssh.