If you go to System Settings - or type "user" in the dash - you should see an icon there for user settings.
If you are the administrator, which you most likely are unless you're using Ubuntu on a network, say at work, your own login is enough. Click on that icon to open the follwoing window:
Click on the Unlock symbol in the top right corner to enter your credentials (administrator rights required):
Then click on the + symbol on the bottom left side to enter a new user as this:
Choose the account type "Standard" or "Administrator".
The account will be disabled until a password was given. To do so click on "Account disabled" next to password:
Then enter a password for the new account:
Log out and you will have the choice now to log in as that user.
Normally you will be the admin user and since Ubuntu doesn't give you root but rather allows you to run with root permissions via sudo, it isn't meaningful to have root as a user to log in.
You can create a password for root. Not sure if that's worthwhile, as Ubuntu specifically avoids that path.