You can prove U.S. citizenship (and thus apply for a passport), if you have an original birth certificate from the United States, an old (but undamaged) passport, an original certificate of citizenship or an original certificate of naturalization.
If you changed your legal name, you can get a passport with a marriage certificate or a name change court decree.
If you were born abroad and one or both of your parents were American citizens, you can claim citizenship and get a passport. You will need proof of your parent's citizenship, a copy of their marriage certificate and a copy of your birth certificate.
Children born abroad and adopted by U.S. parents may apply for a passport, provided they are under 18, are residing in the United States, have proof of their parents' citizenship and their adoption is final.
Delayed Birth Certificate
A delayed birth certificate--issued more than one year after your birth--may be used to get a passport, provided you also have proof of where it was created. In addition, you need to have it signed by the doctor who oversaw the birth, show public records of your birth or have an affidavit signed by your parents.