It is recommended that once you arrive in your host country, you register with the U.S. Embassy or Consulate. The "Smart Traveler Enrollment Program" (STEP) is a free service to allow U.S. citizens and nationals traveling abroad to enroll their trip with the nearest U.S. Embassy or Consulate. Registration makes your presence known, and enables a consular official to contact you in case of an emergency. Registration is especially important for those who are staying in a country longer than one month, and/or who will be traveling to a country that is experiencing civil unrest, has an unstable political climate, or may experience a natural disaster such as an earthquake or hurricane. Your whereabouts will not be released to inquirers without your written authorization.
Keep Embassy or Consulate information inside your passport for reference.
Read more about registering via the STEP program at the Department of State website.