Under U.S. law, all male U.S. citizens (even dual citizens) between the ages of 18 and 25 must register with the Selective Service System. You must register within 30 days of your 18th birthday. Registration can be done online. The same law also requires all male U.S. citizens, age 18 to 25, who are already registered to report any change of address within ten days. This too can be done online.
It is important to know that even though a U.S. citizen registers with the Selective Service, he will not be automatically inducted or drafted into the military.