Stock for any product is managed under stock tab for the product. If stock is disabled here, then enable it.
Here we can update the stock quantity for the product.
You can also update the stock location, if stock is being managed at more than one location.
You can mention the stock reduction strategy to decide when stock for the products must be reduced by the quantity of order also you can setup the minimum stock quantity at which you should be notified about the depleting stock so that it can be replenished.