MS SQL Creating Tables and Adding Values by a Trigger

I have a table for products. They have product name, id, and quantity. By using a trigger whenever I insert a product to this table for example id =145, quantity= 10 I want that existing table get updated and the quantity of the item with an existing id will be decreased by one and autmatically by the trigger we will create a new table only 2 columns, id and quantity, and it will be id=145, quantity =10. How can I do this?