you can apply at the local USPS office, just ask them for the Selective Service form. Usually its on the wall anyways, Don't put SSN info. And no, ICE will not come to your house because you filled it out.
I filled out my SSS about 2 years ago, still in the USA, still undocumented...
Initial Approval- 12/13/2012 - Vermont
Renewal Sent- 8/4/2014 Notice 8/11/2014 - Nebraska. Bio letters 8/28 for 9/11 @ 11AM