Hi Rakesh,
How to define the structure of the segments of idoc. Do we need to give the external system the structure we want or will they give the data structure they are sending ?
You need to prepare mapping with third party(middleware if there) to maintain source and destination fields mapping. It will contain details like source field will be which destination field, what will be relation type(1:1 , 1:N etc).
Is it mandatory to define parent child relation between segments for data coming from non sap system.if yes. How ?
No its not mandatory. It depends on relationship of data. Take an example of PO. A PO can contain multiple line items so a PO idoc will have a one header and multiple child items under it.
There are many resources available on the net, so you can learn how to create segment/idoc type and relevant mapping.
You can start from here:
Defining and Using a Basic Type - IDoc Interface/ALE - SAP Library