C# session, insert into database, web service

Question

How do you session a gridview containing data (eg. ProductId, ProductName, ProductImage, ProductDescription, ProductQuantity, StockLevel) <-- the data is taken from supplier side using a web services by the supplier side and so the company consumed the web service to show all the products display..
However, for the product quantity, user can choose how many items they want to buy, using a dropdownlist filled with values (eg. 20,40,60..) dropdownlist is being added in gridview using item template and the values, just by setting the values in the edit template something..There is also an add to cart button. The add to cart button is a button to carry / session all the data chosen to the nxt page and be displayed into a gridview and then be inserted into the database using a web service.. How can I do all these?? I need steps and coding samples thanks!
Thanks for your help! If needed clarifications, I can send you an example of my ui. (: