To build a robust Order form in zoho creator you need to be able to automatically "fetch" the information about a product and let the system pre populate the subform with pre existing data in the Products Table.
First, you need to have a Products form with all the relevant information about the Product. Price, Description, etc.
Second, you need to fetch the Product information based "On user input" inside the items Subform and add the below deluge scripting.
prodct = Products [ID == row.Product];
row.Description = prodct.Description;
row.Price = prodct.Price;
row.Qty = 1;
if ((row.Price != null) && (row.Qty != null))
row.SubTotal = (row.Price ...