Use the usermod command to add the user to the sudo group.

usermod -aG sudo username

By default, on Ubuntu, members of the sudo group have sudo privileges.

As the new user, verify that you can use sudo by prepending sudo to the command that you want to run with superuser privileges.

sudo command_to_run

For example, you can list the contents of the /root directory, which is normally only accessible to the root user.

sudo ls -la /root

The first time you use sudo in a session, you will be prompted for the password of the user account. Enter the password to proceed.

Output:
[sudo] password for username:

If your user is in the proper group and you entered the password correctly, the command that you issued with sudo should run with root privileges.