How can I get the root terminal to add myself into sudoers file so that I can act as root?

Once again, I'm trapped in a Linux security circle. When I start terminal, this is what I get:

So I looked up the internet how to solve it. All solutions unsurprisingly require root permissions. But wait, I can't use root because I'm not in the sudoers file! So how to get out of this circle?

I'm using Debian with XFCE.