Generally, United States citizenship is acquired either at birth or through the naturalization process.

Citizens at Birth
"Citizens at birth" are those citizens who were born in the United States, or who were born abroad and acquired their citizenship by descent through their parent or parents.

Born in the U.S.A.
If you were born in the United States, you are automatically a U.S. citizen, unless your parents were foreign diplomats. In most cases, citizenship is also granted to those born in Puerto Rico, Guam, and the U.S. Virgin Islands.

Born Abroad with Both Parents being U.S. Citizens
Generally, even if you weren't born in the United States, you may still be a U.S. citizen at birth if both of your parents were U.S. citizens at the time of your birth, and at some point in their lives, either or both parents lived in the United States.

Born Abroad with One Parent being a U.S. Citizen
If only one parent was a U.S. citizen and you were born after November 14, 1986, you may still be a U.S. citizen at birth if your citizen parent lived in the United States for at least 5 years before you were born, with at least 2 of the 5 years of living in the United States after his or her 14th birthday.

If you were born before November 14, 1986, your U.S. citizen parent must have lived in the United States for at least 10 years before you were born, with 5 of the 10 years of living in the United States after his or her 14th birthday in order for you to be a U.S. citizen at birth.