By default root does not have a password and the root account is locked until you give it a password.
When you installed Ubuntu you were asked to create a user with a password. If you gave this user a password as requested then this is the password you need. If not, or you have forgotten it, then you need to set a password.
Instructions on how to do it are here:
How do I reset a lost administrative password?
Once you have an administration password you can use
sudo to run commands that would otherwise require you to be root and enter your password.
If you want to set a root password see this question
How to enable root login?
However, read all the answers carefully first to make sure you want to do this. It's not recommended and many of the answers explain why.