Hello Arun,
BAPI performs a functionality of a transaction or some part of functionality of transaction where data can be posted/created without user continuous interaction or without screens.
I would suggest you if you are planning to use a BAPI, then understand the functionality:
You may go through the documentation completely, identify the transaction which is used for same process and interpret the same by using it or provide inputs to it. You will have better and proper understanding of the functionality and will come to know mandatory fields, the different error messages you would get. In your case try using transaction VA01 and check it out.
Also in BAPI's mostly for creation/change of data or some document, you need to pass flags such as 'X' or 'U' or 'I' for it to be executed completely in tables ending with X eg.order_header_inx[].
Hope it is helpful in some way.
Thanks.