This is the 11 digit number on the electronic I-94 or the number on the white card stapled in your passport. If you don't have the electronic I-94 then visit https://i94.cbp.dhs.gov/I94 to retrieve the number
If you don't have a Social Security Number, you will first have to apply for a Social Security Number. Visit International Services for instructions on obtaining a Social Security number.
For example: If you are graduating in the Fall or Spring say Fall 2013 or Spring 2014