You need to call Selective service office and they ask them to provide you a copy of "Request for Status Information"
Letter. Its a letter that states you were ineligible to file for Selective service at the time you became PR.
When filing for N-400 leave that section blank ( n/a), you will be highly likely to be asked about the Selective during the time of your interview. At that point of time, show officer that you were exempted form selective service with the proof ( status information letter from Selective service).
Good Luck with the N-400 filling. I know its a hassel, but you will be all right.
25 years old Male single
United States Citizen