I was born outside the USA. I believe I have a claim to US citizenship. How can I prove it?

If you have a "Consular Report of Birth Abroad FS-240" or "Certification of Report of Birth (DS-1350)" that's all you need. You can apply for a US passport at any time, either in the USA or at a US consulate abroad. If you do not have one of these documents, you will have to get one from the US consulate in the area near where you were born if you are still under the age of 18. If you are an adult and your birth was registered at a US consulate but you have lost or misplaced the document, you can make a written request to the Vital Records Section in Washington DC. If your parents never requested a Report of Birth Abroad for you and you are over the age of 18, you can apply for a US passport directly at a US consulate and/or a Certificate of Citizenship (in the USA). A US passport is definitive proof of US citizenship.