Need help creating an affiliate marketing program for my Ecommerce store

I have a website that directs my customers to a fitness software program (Trainerize). I want to build an affiliate program where I can track who referred the client, which is necessary only if the client has completed a purchase. I would like for this to be done using either a promo code or custom link.

If this project is completed in a timely manner the Full Package Fitness team will provide a hefty tip :) $$$