App to log activity / usage / payments

[jobreu]

I am trying to figure out the best way to do this, any ideas would be appreciative. 

I want to create an app for my business to record usage on devices and payment totals for customers. 

So It's basically the following, the customer comes in and takes a device, brings it back 2 3 weeks later. I add to that customer his total usage and the total paid. I have both new and recurring customers.  

I have been sitting on this for weeks already and couldn't come up with the basic concept on how to do this...