I went into my tax internship having a fairly good understanding of individual income tax and was taking my corporate tax class at the same time, but my understanding was that they don't expect their incoming interns to have any tax experience or specialized knowledge. It certainly helps, but definitely not required. But if you want to brush up, check some courses on Coursera. They're usually free if you don't care about getting a certificate: https://www.coursera.org/learn/federal-taxation-individuals
https://www.coursera.org/learn/federal-taxation-business
https://www.coursera.org/learn/taxation-business-entities-part-1
https://www.coursera.org/learn/taxation-business-entities-part-2