If you believe you are a U.S. citizen, you'll want a document to prove it.
If you were born on U.S. soil (a U.S. state or a U.S. territory, meaning Puerto Rico, the U.S. Virgin Islands, or Guam) and there is a record of your birth, a standard U.S. birth certificate issued by a state government is your primary proof of U.S. citizenship. (Birth certificates issued by hospitals are not official records and do not serve as proof of citizenship.)
If you were naturalized in the United States, you will have a naturalization certificate.
However, if your birth took place outside the territorial United States and you have a right to U.S. citizenship through your parents, you will not have either of these documents. ( Such rights are discussed in detail in the article U.S. Citizenship by Birth or Through Parents.) In this case, you will have to apply for either a U.S. passport or a certificate of citizenship.
If you were born abroad to U.S. citizen parents, you can apply for a U.S. passport in the same way as someone born in the United States. However, you will have the added requirement of establishing your citizenship claim. The evidence you'll need to have on hand may include:
Evidence may take the form of birth or citizenship records, work or tax records, or affidavits from you (and perhaps even from your parents or grandparents), for example, explaining why you were unaware of your claim to U.S. citizenship.
Passports are available from passport offices in the United States or at U.S. consulates abroad, but experience shows that you have a better chance of succeeding by applying to a U.S. consulate.
You can also get proof of your citizenship by applying for a certificate of citizenship from U.S. Citizenship and Immigration Services (USCIS, formerly called the INS). Anyone with a claim to U.S. citizenship can apply for a certificate of citizenship. Citizenship certificates are issued only by offices of USCIS located inside the United States.
In most cases it is more difficult to prove your citizenship through a certificate of citizenship application than by applying for a U.S. passport, mostly because it takes more time. (In some of the busier USCIS offices, it can take over a year to obtain a certificate of citizenship.) However, if your U.S. citizenship was obtained automatically through the naturalization of a parent, a certificate of citizenship application is your easiest and best choice, because the evidence needed to prove your claim is usually obvious and easy to acquire.
Evidence of your claim to U.S. citizenship should include your parents' birth certificates, marriage certificates, and naturalization certificates. You will also need your birth certificate, marriage certificate, or divorce decree to prove what your name is and to document any changes to your name.
If you were born outside the United States and your parents were U.S. citizens at the time, they may have registered your birth with a U.S. consulate. If they did so within five years of your birth, they would have been issued what is called a Consular Registration of Birth Abroad. The consular registration is conclusive proof of U.S. citizenship.
But, if your parents did not take the steps to register your birth with the consulate before you turned five years of age, there is no way of obtaining one now. Also, there is no way to obtain duplicates if your parents lost the original and any copies they received at the time of your birth. You will have to apply for a passport or certificate of citizenship using the procedures outlined above.
For help applying for a passport or certificate of citizenship, see U.S. Immigration Made Easy, by Ilona Bray (Nolo).