Hi,
I just got a job under OPT that will pay me $45,000/ year. I am from India and have been in US since August 2008. My question is that what is the state tax in NJ for OPT job and federal tax for OPT job? I believe I don't have to pay Social security and medicare. Any answer will be much appreciated.

You are correct about NOT paying Social Security and Medicare taxes; make sure your employer is aware of that fact.

You will pay federal and state income taxes, however. If you are single, you are in the 25% marginal tax bracket for the federal tax.

For New Jersey, you are in the 5.525% tax bracket.