Selective service

How much trouble do you get into if your eighteen and you haven't registered?

May prevent you from getting certain jobs down the road.

uhh sorry I dont know :{

If you do not register, you could be prosecuted and fined up to $250,000 and/or be put in jail for up to five years. Registration is also a requirement to qualify for Federal student aid, job training benefits, and most Federal employment.
http://www.sss.gov/QA.HTM#quest4
Also see
http://www.sss.gov/FSbenefits.htm
http://www.sss.gov/fsstateleg.htm

Odds are you won't get in any trouble at all, since failure to register has never been prosecuted.

However, all government employees as well as many employees of companies that contract with the government have to prove compliance.

The simple thing to do is to comply until there's a real cost to you. Register, but if they reinstitute a draft and you get called, just don't go.