Green card - SSN - Non citizen question

Hi everybody

Heres the story, I live in Mexico, my Dad lives in the USA - NJ, they got divorced long ago. and Im a Mexican Permanent Resident Alien. Now I lost Green Card and SSN. I arrived today to the USA, with a letter/permission to travel to the USA in order to get my issues fixed.

I went already to the USCIS page and read about the I90 doc, but i dont have/know my SSN. what should I do, right now im at my Dads eho is a citizen now. I beleive I need my SSN first in order to recover my GC but Im not form the states and I have nothing to prove i am a permanent resident..

please help!