The ESTA is only for foreign tourists travelling to the US - it's a 'visa waiver' that is issued to people from certain countries instead of a visa, and it gives them permission to come into the US as a visitor for up to 90 days.
As you are a US citizen, you don't have to apply for permission to enter your own country, so you don't need an ESTA - you just show your US passport to immigration when you arrive.
