This is probably obvious to you, but as a novice linux user, I am wondering if I am doing anything wrong.
I installed Clear in virtualbox and created my user once installed (in gnome).
I can login without problem to my Gnome desktop, but when I try to SU in my terminal, it says the password is wrong.
Have I done anything wrong or am I missing something?
Thank you!
sudo will let you run a command with elevated permissions using your account and password.
su will let you change to the root user, so you need to set and use the root password. You probably need to set a root password first by switching terminals (CTRL + ALT + F3) and to login as root. It will prompt you to set a password.
Thank you so much! I learn every day 
FTR An easy alternative that doesn’t require you to go to a text console is, permitted you are in the wheel group, to do sudo passwd.
sudo -i
will act just like su (leaving you with a shell you can run multiple commands with).