The simplest procedure is to boot from an installation medium, "live CD" or anything like that, find the partition where the system is installed, and change the hash of the password to a blank. You are looking for the file "/etc/shadow" in one of those partitions. It should be easy to mount the partition; look up instructions online. Edit the file with your favourite editor. Make the second field blank. It is the one after your username, and fields are separated by a colon. When you reboot from the regular drive as usual, you can login without a password.