Registering with the Selective Service before you apply
From USAJOBS Help
By law, young men are required to register for the Selective Service to ensure our Nation has a structure and a system of guidelines for a prompt, efficient, and equitable draft, in the event of a crisis. Registration for the selective service ensures you to stay eligible for:
Registration Is Easy and It's the Law Register with the Selective Service! Save a stamp. Save time. Register on-line at http://www.sss.gov, or pick up a registration card at your local post office.
Almost all males living in the United States, between the ages of 18 through 25, are required to register with Selective Service. It's important to know that even though he is registered, a man will not automatically be inducted into the military. In a crisis requiring a draft, men would be called in sequence determined by random lottery number and year of birth. Then, they would be examined for mental, physical, and moral fitness by the military before being inducted into the Armed Forces, or deferred or exempted from military service.
WHEN TO REGISTER?
The law requires virtually all male United States citizens (regardless of where they live), and male immigrants residing in the United States (permanent resident aliens), to register within 30 days of their 18th birthday. Therefore, to be in full compliance with the law, a man turning 18 should register during the period of time beginning 30 days before, until 30 days after his 18th birthday . . . a 60-day window.
Late registrations are accepted, but not once a man reaches age 26. Men who do not register within the 60-day window are technically in violation of the law and should register as soon as possible.