A President must be born in the US, be at least 35, and be a resident of the US for at least 14 years. If they are not born in the US, they can still be considered natural born citizens if either or both parents are citizens of the United States.
No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.
According to US code Title 8, Chapter 12, Subchapter III, Part I, Sections 1401-1409 (
http://www4.law.cornell.edu/uscode/html/uscode08/usc_sup_01_8_10_12_20_III_30_I.html), a person born in the US, Puerto Rico, Panama, Alaska, Hawaii, the Virgin Islands, or Guam are natural born citizens, or, if not born in the US, if one or both parents at the time of birth are US citizens, the person is a natural born US citizen, and is therefore eligible to be President.