Raspberry pi 3

1) user and passwords:

Distribution   | Username   | Password
---------------|------------|-------------
Debian Squeeze | pi         | raspberry
Arch           | root       | root
QtonPi         | root       | rootme
Raspbian       | pi         | raspberry
OpenElec       | root       | openelec
Pidora         | root       | raspberrypi
RISC OS        | n/a        | n/a
raspbmc        | pi         | raspberry

2) add users

$sudo adduser username

$usermod -aG sudo username

3) Ports

$lsof -nP -i | grep LISTEN
$netstat -tlpn | grep LISTEN
 

4) vncserver (reinstall, by default sometime doesn't works)

$sudo apt-get install tightvncserver 

5) Firewalls

iptables -I INPUT -p tcp --dport 5900 -j ACCEPT
iptables -I INPUT -p tcp --dport 5901 -j ACCEPT

Resources:

[1] https://www.digitalocean.com/community/tutorials/how-to-create-a-sudo-user-on-ubuntu-quickstart

[2] direct way https://www.shellhacks.com/how-to-grant-root-access-user-root-privileges-linux/

[3] vncserver https://quaintproject.wordpress.com/2013/03/24/establish-a-vnc-connection-to-your-raspberry-pi-from-a-linux-pc/