BAPI Help

Hello Guru's

I am new to SAP and i am trying to learn BAPI's. So can anyone please guide how to create BAPI's. For example i want to create BAPI_SALESORDER_CREATEFROMDAT2. I am not able to understand the keys or how to input data in the fields. Sorry for such a generic question.

PS: i even searched the forum for this topic but was not able to understand where to start.

Thanks,

Pranava