
The I-94 is the card you should have been issued when you entered the U.S. An image of an I-94 is available here:
http://www.miracosta.edu/studentservices/iip/newstudents.html
If you do not have the I-94 any longer, then you should write back to USCIS and tell them that you lost it. In order to apply for permanent residency ("green card"), you have to show that you were "inspected and admitted" by an immigration official. The I-94 is the easiest way to do that.