If you have moved to the USA and going through the green card process make sure you register for selective service. When I moved across a couple of years ago I remembered reading on a form that I was automatically entered into selective service. So I did not do anything. This is not the case. If you are under 26 years old, you have to go register. www.sss.gov has the info or go to a local post office.
If you don't do this, you will be unable to apply for US citizenship. Fortuanately I was looking through the citizenship forms a few weeks before my 26th birthday and noticed. So I managed to mail in a form and get registered just in time. In fact it was processed on my 26th birthday. So I got the confirmation number and was on selective service for less than a day. As you are removed the day after your 26th birthday.
It is nothing to worry about as no one on selective service has been called up since Vietnam.
