Hi,
Enter the stock of each product in the "quantity" field of the products. HikaShop will then decrement that value automatically on each sale, either at the creation of the order, or at the confirmation of the order (based on the "Update the product stock on confirmed status" setting).
On the listing of products, you can see that stock for each product in the quantity column. And if you want to search for one product with a bar code reader, select the search box, scan the bar code with your bar code reader, and that will fill it with the product code automatically, and press enter and it will launch the search.