INBOUND program For custom IDOC

Hi ,

i have a one senario on creation of IDOC,that is interface for the transmission of the Acknowledgement of check Remittance and Supplemental interface,once the message is received Successfully through OUTBOUND program it triges the Acknowledge other wise it send one mail to respective team for that i need to create a one inbound program please explain me i am new in the area of ALE/ IDOC please give in deatials.

Thanks,

Harinath