I'm going on an internship in the USA this Summer. I already received my J-1 Visa but I was wondering if I still have to apply for ESTA. Is this the case?
Visa holders do not need to apply for ESTA to travel to the US for the purpose of their visa.
If you intend to travel to the US for a different purpose than the one permitted by your visa, i.e. you want to go on a holiday but not start your exchange program, then you need to apply for ESTA to travel under the Visa Waiver Program. (But if you do that, as I understand it, you have to exit and re-enter the US to change to J-1 status.)