can anybody help me how can i add quantity of my products while adding inventory items ??
You can either add inventory when entering your inventory opening balances, first day of business, or when you purchase the inventory, after the first day of business. For the opening balances read this guide.
@Brucanna did not mention that in addition to opening balances, payments, and purchase invoices, you can also add manufactured inventory items through production orders. A production order produces a finished inventory item from raw material or component inventory items already in the system, adding quantities of the former and decreasing the latter.