United States citizenship can be acquired through birth or through naturalization.
Acquisition by Birth
The 14th Amendment of the United States Constitution grants citizenship to everyone born in the United States. This includes US territories: Puerto Rico, the Panama Canal Zone, the Virgin Islands, Guam, and the Northern Mariana Islands.
People born abroad my acquire US citizenship through their parents, if at least one is a US citizen, but this is subject to a number of conditions, including the age of the person, their parents’ residence in the United States and civil status at time of birth, and whether both parents are US citizens.
A certificate of citizenship may prove your US citizenship if you were not born in the United States or do not have a US passport.
Naturalization
Legal permanent residents may become citizens through naturalization. Normally you need to have been a permanent resident for five years, although there are a number of special cases.