Hi Nandhini
Yes Postman can be used as testing tool for the API that you are developing in your WEB.API application , you can do the same by running API application with swagger also.
As per the good practices you should have Models while working with API calls.
So that when you return any responses from the API calls you will have standard pattern of giving the output
For example :
public class CartDTO
{
public int Id {get;set;}
public int Username{get;set;}
public decimal Fee{get;set;}
}
public class ResponseDTO
{
public bool IsSuccessful{get;set;}
public string Message{get;set;}
}