The ESTA (Electronic System for Travel Authorization) is a scheme for visitors to the United States from countries in the Visa Waiver Program.
It is an entry requirement for certain non-US citizens only. American nationals do not require any additional travel document; they need only their American passport to enter the United States. Therefore, if you are an American citizen, you do not need an ESTA to travel to the US.
Only nationals of the 39 sovereign states in the US Visa Waiver Program are eligible to apply for ESTA travel authorization to travel to the United States.