How to set up an automatic subtraction formula for monthly payments?

I would like to find a formula to subtract automatically a monthly regular payment from a specific amount.

ex: loan of 12 000$ with monthly payments of 400$. I would like a formula that will keep track automatically at the beginning or at the end of each month how much I still owe in total.

loan 12 000$ in August 04, first monthly payment of 400$ in September 04. How much do I still owe today ?

thank you