Alessandro,
when you are receiving an IDOC then it comes to MII in Message service. So you have to read the IDOC from there. Now there have two way to read data from message service.
1) Using Category based processing rule
2) Using Transaction based rule
For both the cases you have to define a processing rule (Message Services --> Message Processing Rule from MII admin pages).
If you want to create a Category based processing rule then you have to define a category and have to assign it to the listeners
If you want to create a transaction based processing rule then you have to assign the transaction name by which you want to process and have to assign it to the listener.
Now,
if you are using category based processing rule then you have to create a BLS and have to call that BLS from the scheduler by some interval. and within the the BLS you have to use "Query Message" action block to get the message ID and then have to read the msg by "Read Message" action block for that same message ID. then you will get the IDOC XML in your BLS.
Hope it will help you.
Regards,
Suman