Delphi: Capture keystroke anywhere in the program

I have written a program in Delphi, in which I want to run some commands when the user presses a key. Therefore, I use the following procedure in the OnKeyPress of my form to capture the key press:

procedure TForm1.FormKeyPress(Sender: TObject; var Key: Char);
begin
   if key = 'a' then ...;
   if key = 'b' then ...;
   if key = 'c' then ...;
end;

However, this does not work at any time. Sometimes the function is executed, sometimes it is as if the button was never pressed. Does anyone know, what to do and how I can achieve a program-wide keypress?