How to customize purchase order smartform BBP_PO in SAP SRM 7.0?

Hi friends,

I am new to SAP, as well as to SRM world. Currently I have a object with me from purchase order smartform in SRM. My job is to copy the standard PO form named BBP_PO into "Z" form and to make customizations in the copied one. I don't have idea about how to proceed with it. Please give me some idea about required processing classes, methods, BADI's, FM's etc,.. Can anyone please tell me how to do this job? I am a fresher so if you help me in this means, it would be very helpful for me and for my future.

Thanks and regards,