301 - Nationals and Citizens of the United States at Birth
There are four ways to obtain U.S. citizenship: 1. By birth in the United States or its territories; 2. By birth outside the United States to a US parent; 3. By naturalization; or 4. By naturalization of a parent while the child is under 18 years old.
Section 301(a) of INA discusses in more detail about items one and two above.