One might derive citizenship because one or both parents were born in the United States. There are residency requirements prior to the child’s birth that one or both parents must meet. The length of time depends on when the child was born (because there were several laws passed modifying requirements over the years), and whether one or both parents were citizens when the child was born.
One may also derive citizenship through the naturalization of one or both parents, if the naturalization occurred prior to the child’s 18th birthday and the child was lawful permanent resident prior to age 18. Both forms of derivative citizenship have specific requirements depending on when the child was born, and an immigration attorney should be able to analyze your case and determine whether or not you have a derivative citizenship claim.