There is on-line registration with the state department gov site. Or you can walk into any embassy to do it in person. We lived overseas in different locations for many years and it was normal practice for us to register each time [back them no on-line, did in person]. This is helpful for many reasons. They know where to find you in case of emergency, your passport is registered and on file in case it's lost or stolen, you can get travel and/or warning updates.