If your data was migrated from your previous Mac, your password should still work. If from a PC, unless you specifically offered input on the password you wanted the store to use, you may not have a password. To test this theory, go to System Preferences->Accounts, and click the padlock in the lower left. Press enter/return without entering anything and if the padlock opens, then do the same for any other password prompts you get from the system. You can add a password by clicking the Change Password button. Note that you may have to re-enable Automatic Login (from Login Options) if you don't want to have to enter a password when you turn on the computer.
