To add a user to your system’s sudo user list, do the following in Ubuntu:
Type visudo on your command line, of course you will need root access to do this.
Find the section that looks like this:
# User privilege specification root ALL=(ALL) ALL
Add your user straight after the above code like this:
# User privilege specification root ALL=(ALL) ALL newuser ALL=(ALL) ALL
This is the most basic way to add your new user to the sudo list.