United States law requires that all eighteen year old men must register with the Selective Service. In the event of a national emergency, it is the responsibility of the Selective Service System to provide untrained manpower for the Armed Forces. Congress has not declared a emergency significant enough to order a draft since 1973. In the event that a draft was ordered, the selective services would conduct a birth date lottery. Those who turn 20 years old would during the calendar year selected would be called up first. If more men are needed, those 21 to 25 would then be called, starting with the youngest. At this time, the United States government only takes the names of men, thus women may not register. By not registering with the Selective Service, you are breaking the law, and could be sent to prison for 5 years, and fined $250,000.
To register or for more information, go to the selective service website at http://www.sss.gov.