Calculator in Delphi 7?

Can anyone tell me how can i make calculator like the windows calculator
that you can to press the keys (from 0 to 9) and (+, -, *, /).
I have the code made in Delphi 7 but with event OnClick.
Can anyone know how to make with keypress. My professor told me that
I need to declare one more variable in Public Declarations but don't tell me how
to make.
Please download the source http://www.megaupload.com/?d=6J6NHNWZ and if anyone know please write the code you can
in the same project and upload again or send me by e-mail at
ivica_miloshevski@yahoo.com
if anyone have a calculator in Visual Basic or C++ you can send me that
but with KeyPress.
Thanks a lot!!!!