NO SOCAIL SECUIRTY TAXZES WITHHELD ON MY PAYCHECK

I just started a new job and never filled out a w-4 form. it was done for me. i got my first paycheck and only medicare taxes were taken out. It says i am exempt from FICA taxes, but i dont know why I would be? I dont work for a gov agency etc. How can I make sure to get those taxes taken out of my pay check?