Question about selective service registration?

I have no knowledge about that , and today I received a letter saying that I am registered. I am kinda shocked , astonished and surprised at the same time. I am a 20 year old male. I NEVER registered for anything , I didn't even know you have to do it. I live in Us for like 7 years , I have SS number and green card. My question is , how can I be registered for it, when I didn't even fill any application , much less for military service. I was about to throw that letter away until I saw that I have to keep the selective service number. Ok , so another question, is there thats all ? Will I be called for like some military commission or something? I come from Poland and a year ago I my grandmother received a letter for me saying that I have to show up for a military commission so I wasted like 2,000$ and flew there for 2 weeks. Even thought there is no more draft there I had to waste $ to go and meet some old dudes for basically nothing.