Taking Count of my Items

i am new to samba and i want to be very familiar with Samba… i want to take stock of my inventory, i.e if i have 50 Fanta 75Cl, if i sell one out of it i should have 49left and on like that, this will allow me know the number sold for a period of time and how many i have left

any help Please

Check this link out:

I think there are some in links as well, good luck - it is actually a lot easy than it looks so have a go!