Management Of Payments

First, ignore the colored portions. This is how I kept track of what I had completed in the form as I created it. Clients attend class once a week and pay once a week for their classes. I'm taking this one step at a time. At the initial interview of a particular client we will determine the payment per session due, and we will enter this amount in the column labeled "payment per session. We also need to keep track of how many sessions they have paid for. So in short, I'm looking for the following:

When "amount paid" equals "payment per session" set "minimum payment due" to $0 and increase "classes paid" by +1

I figured out how to enter the payment and return the minimum payment due to "0" but how can I make the "classes paid" increase by 1 every time the minimum payment equals "0"?