If you have forgotten your password, you change your password via the standard Salesforce functionality.
Click the "Forgot Your Password?" link at the bottom of the log-in screen. This will take you to another screen where you can enter your username and receive an email which contains a link to reset your password.
If you have also forgotten your username, you can contact your system administrator to manually send you a password reset email. This is done by locating the relevant user record (Setup>users>"Full Name"). The "Reset Password" button can then be clicked to send the email.