Boot your machine up while holding down the Command and "S" keys. This should take you to single user mode. Type in the following lines and press enter:

/sbin/fsck -fy /sbin/mount -uw / launchctl load /System/Library/LaunchDaemons/

Now you can change the user's password. For example, if the user was called awesomeman, you would type in:

passwd awesomeman

It will then prompt you to enter the new password twice. Once finished, just reboot your machine:

shutdown -r now

That's it! You should now be able to boot up and log in with the new password.