Logon Vending Machine - Purchases Deducted From Balance

I am making a Vending machine that requires you to logon and then you can purchase things and the purchases are deducted from your balance. The logon information as well as the balance is stored in an access file. I have the logon working well and understand everything except the things dealing with the balance. How can I make it so that when you logon it takes you to the purchase screen and The person you logged on as name and balance are shown on labels. I also don't know how to deal with balance. So when they log on and purchase the amount is detracted from their particular balance.