You should enter the username and password you set during installation.
You could try:
Username: root
Password: linux
If you log in to Ubuntu with this log in, CREATE A NEW USER IMMEDIATELY, LOG OUT, THEN INTO THE NEW USER ACCOUNT!
NEVER, unless absolutely necessary, log in as root.
Or:
If you can't remember the username or password AT ALL or the above doesn't work, reinstall and set a username and password you can remember.