I Just Dont Have A Clue Of How To Do It

Im trying to make a simple database in access , so far all is going well but im stuck with the inventory.
I have "products" table with Id [autonum],name[text],price[money], provider[number], stock[number]
Providers table with Id[autonum] ,name [Text],tel[number],
Employes table with Id[autonum], name[text],tel [number]
and i have Sales table with Id[autonum],date[date],employer[number],product[number],quantity[number],total price[money]

I have relationships between table.provider and provider.name then sales.employer with employer.name and sales.product witht products.name.

Now what i want to do is that every time that i make a sale it modify my stock number on the products table. I think the proper way is just to calculate stock every time i need it but i dont know how to do that either :D
Ill upload my db but i have to tell you that is in spanish so if you haveany question im here to answer.Thanks a lot to all