Hi there Stephanie,
You will need some software that is going to track affiliate sales.
You can get a product for your own site, or use one of the affiliate manager systems.
e.g. clickbank, paydotcom.com .....
I use and recommend JVmanager (Fantasos) from John Delavera.
the original JVManager can be purchased for $97 and works very well.
and there are others idevaffiliate ....?
I am happy to help more with JVManager related questions.
cheers
Eric.