Restrict Root Access to server

Adding user with root privileges
sudo useradd -m testuser
sudo passwd testuser
sudo usermod -s /bin/bash testuser

Restart ssh
/etc/init.d/ssh restart

Check your access can ssh but not able to have root access
Open the file sudoers  
vim /etc/sudoers

Add the following lines below the root line in the file
testuser ALL=(ALL:ALL) ALL
write quite

Service ssh restart
/etc/init.d/ssh restart

Check again for root access. Got access.

Changing ssh port. Edit sshd_config file
vim /etc/ssh/sshd_confing
Port 22
change it to desired one

wq

service ssh restart

Check again with new port

ssh -p 220 testuser@SERVER_IP

Successfully logged in.

Restricting root user from direct login.
Edit sshd_config and add

PermitRootLogin no

and comment PermitRootLogin yes

service ssh restart



Now adding sftp users

create backup of sshd_config
Create new users with home directories
Create their passwords
Add it to group