I have read all the replies. Still not quite sure what I should do. My visa changed from F1(OPT) to H1B on Octobor 1, 2007. my employer never deduct anything for medicare and social security taxes from my pay. I just noticed that from my W2 form. So now my question is how do I pay IRS? How do I figure out how much i owe IRS? HR in my firm refused to do anything for me. Btw, I have already filed the tax extension form but I didn't take into account the social security and medicare taxes I owe.  Please help! Urgent!