One of the first things I install on a server is sudo. It’s pretty well documented that it isn’t wise to always use su. I’ve also noticed using sudo forces me to constantly double check what I do so that’s a good side effect.
Get into superuser mode, install sudo.
su apt-get install sudo visudo
Add the user you want into sudo by replacing ‘newuser’
# User privilege specification root ALL=(ALL:ALL) ALL newuser ALL=(ALL:ALL) ALL