In Linux, by default, the first user's account is an administrative account, so if the UI is prompting you for a password it's probably that person's user password. If the user doesn't remember their password you need to reset it. To do this you need to boot into recovery mode.
Reset a lost administrative root password
Restart the system, and after the BIOS screen, hold down the Shift key. You will then be prompted by a menu that looks something like this:
Select Advanced options for Ubuntu
Now you will see this menu:
Hit the down arrow until you select the 2nd entry from the top (the one with the recovery mode in the description) and then hit Enter.
Now you will see this menu:
Using the arrow keys scroll down to root and then hit Enter.
You will now see a root prompt, something like this:
Reset a lost administrative root password
Restart the system, and after the BIOS screen, hold down the Shift key. You will then be prompted by a menu that looks something like this:
Select Advanced options for Ubuntu
Now you will see this menu:
Hit the down arrow until you select the 2nd entry from the top (the one with the recovery mode in the description) and then hit Enter.
Now you will see this menu:
Using the arrow keys scroll down to root and then hit Enter.
You will now see a root prompt, something like this:
root@ubuntu:~#
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At this stage, you should have a read-only filesystem. You have to remount it with write permissions:
mount -n -o remount,rw /
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Now we can set the user's password with the passwd command.
root@ubuntu:~# passwd root
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@ubuntu:~#
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Type in what you want the new password to be at the prompt. After it's successful reboot the machine and the user will be able to log in with their new password.
