Is this your user password or root password? If you still have access to the root account, you can change any user's password as root without being prompted for the original password. Otherwise you'll need to boot into single user mode. Assuming your boot loader is GRUB, hit "e" at the bootloader screen to edit the kernel command line and append the word "signle" (no quotes) and continue booting. If you get asked for a password, i.e. single user mode is protected, you may need to use init=/bin/sh instead of single to launch a shell directly or boot off of rescue media, mount the root filesystem, and edit the /etc/shadow file directly.
This problem has been discussed several times here, so you might want to check the forum search and Google if you need more help with these (admittedly skeletal) instructions.