How can I delete a password I don't remember?

My step-dad has a Mac HP 2310m computer, and my account on it has a password with the hint pirate. I've tried everything i can think of to log into it, but nothing works. I know a password to one of the administrator accounts (my mom's), but i don't know how to change my account's password, or if i even can through the other account. Can I and if so, how? Thanks!