You will find this tutorial on hundreds of places over the web, but it's never too much for a useful thing to be found on how many places can be posible.
1. Start the computer.
2. Soon after the BIOS messages, pres ESC and you'll have the GRUB menu displayed.
3. Press <E> and in the following menu, use the arrows to choose the second line, usually, the one with kernel = ....
4. Press <E> again and begin to edit the line, by adding the word "single" at the end (keep in mind you can use the word "1". Just that, the "1" character).
5. ESC to go back
6. Press <B> to boot the kernel.
The booting process will end in single user mode, with root prompt (sh#).
Now you can change the password easily, like you would be logged as root.