Open AR?

Hi,

I'm new to SD, FI..

Can anyone please tell me what is the transaction involved if we have to post open AR in SAP?

Please write me some explaination also to understand the following requirement:

The Billing is done in legacy system but the AR will be done in SAP. So when we develop the interface for this, what will be the input data and what the interface will do - for AR?

Thanks,any help is really appreciated..

Uma.