'sudo' is not installed, I can't install it, and it asks if I am root

If you do not have sudo installed, you will need to actually become root. Use su - and provide the root user's password (not your password) when asked. Once you have become root, you can then apt-get install sudo, log out of the root shell, and actually use sudo as you are trying to, now that it will have been installed.

Become root, run su, give your root password, and then run:

apt install sudo

To grant administrative privileges to Mariel, run visudo command as root, and then edit your file as follows:

# User privilege specification
root    ALL=(ALL:ALL) ALL
Mariel  ALL=(ALL:ALL) ALL

Log in as user Mariel. You will be able to run sudo without problems.

As an alternative to given answers, if you do not know the root password to allow you to use su, you can boot to single user mode.

At the grub menu, press e to edit the boot command line. Near the end of the line beginning linux add the word single, and continue the boot. You will be placed at a root prompt and can reset the root password so that you can use su on the next normal boot.