How to edit the standard code for SAP logon screen

Hi All,

My requirement is we are having SAP logon screen which asks for user id and password manually instead of entering those credentials manually they should pop up automatically for this purpose we need to edit the standard code like we have to find the enhancement spots where we can give this userid and password by default...Can anyone please help me out like where i can find standard code for sap logon screen and how to make the changes...