Excerpt from website: If you are a man ages 18 through 25 and living in the U.S., then you must register with Selective Service. It’s the law. According to law, a man must register with Selective Service within 30 days of his 18th birthday. Selective Service will accept late registrations but not after a man has reached age 26. You may be denied benefits or a job if you have not registered. You can register at any U.S. Post Office and do not need a social security number. When you do obtain a social security number, let Selective Service know. Provide a copy of your new social security number card; being sure to include your complete name, date of birth, Selective Service registration number, and current mailing address; and mail to the Selective Service System, P.O. Box 94636, Palatine, IL 60094-4636.
If you have a social security number, you can register online (click here). It's quick and easy.