All men between the ages of 18 and 25 living in the U.S. must register with selective service. Selective Service does not collect information that indicates whether or not someone is undocumented. It is important to register because you could be denied benefits or a job if you have not registered. It is also important to protect yourself for future U.S. Citizenship possibilities.
The law states that all men must register within 30 days of their 18th birthday. Registration is available at any U.S. Post Office, and you do not need a social security number to register.
More information is available at the Selective Service website