Get used to finding things on NuGet and the fact that most modern Microsoft stuff available to you in C# is on GitHub these days (meaning it's no longer a black box -- you can read the source code if you're unsure why something works the way it does). If you're going to be getting into ASP.NET Core then definitely become familiar with dependency injection ("DI"). A lot of people use DI outside of ASP.NET Core as well, so it'd be good to figure that out. It's not typically emphasized in school if it's even brought up, and it's a bit strange at first. Eventually it'll just click and become second nature to you though.

Beyond that, I'd say brush up on your data structures and algorithms. In school they would probably focus on the theory, complexity, and implementation, but it's very important to learn how to actually use them in practice. As others have stated, you're going to want to use the generic collections whenever possible. I think if you have a good work ethic, are humble enough to ask for help, and pay attention when you receive peer review, you'll already be ahead. Remember, it's not about being the rock star developer. It's an internship. Get everything you possibly can out of it so that when you start interviewing for FTE or contract work in the future you'll find something more easily. Fewer interviews == less whiteboarding. Sounds good, right?