Hi Gurdeep.
Customers & Leads are stored in the same Customer table, and it is more of a master table. Contacts are more of a child table. So, in other words, a Customer or Lead could have multiple contacts associated with them.
When you create a customer or lead it automatically creates a single Contact with it.
Best thing for you to do is:
1. Insert a Customer, making sure the field IsLeadStatusOnly is true.
2. Get back the resulting Customer.RecordID of that customer.
3. Query the Contact table for the Entity_RecordID matching that Customer.RecordID. This will give you the Contact.RecordID.
4. Update the Contact with any additional information you want.
I hope that helps.
Paul