If you are a U.S. citizen planning to travel abroad, you must comply with the document requirements for that country. For a list of Foreign Consular Offices in the U.S. visit the U.S. Department of State website.
Each individual arriving into the United States must complete the CBP Declaration Form 6059B. Explanations and a sample declaration form can be found on the Sample Customs Declaration Form.
Find answers at the CBP Info Center.