Be willing to help seniors, managers, or partners n anything they may need. If you're available, let others know you have free time to help out. Be willing to learn and just be yourself. Meet everyone and anyone you can! It's important to network and get to know the people you are going to work with or people in different departments.
I have been at BDO USA LLP for about a month an a half now as one of their core tax interns. Going into the internship the most experience with tax that I had was from my individual taxation course and I volunteered with VITA during the tax season. I think the most important skill you can have going into an internship is the willingness to learn and push your boundaries. You don't need to know how to work all the software and know everything about tax reform because those are things that can be taught throughout the internship. If you are resistant to learning new things, then you will have a difficult time. Going into an internship, you also want to have a positive attitude and personality. One of the things they are look for in interns are people who will fit in and work well with the culture of the company.
Being able to listen to instructions and adapt quickly to software and situations is essential, especially when it comes to utilizing the software appropriately and efficiently. Additionally, being intellectually curious and receptive to feedback is crucial. Most technical skills will come from the on-the-job experience, but exposure to tax concepts will help solidify the "why" and "how" for a lot of the work during the internship. A strong grasp of time management and being able to work independently are also really important in doing well in your tax internship, as you'll be given many projects, and it'll be up to you to figure out how to efficiently use your time to work on them.
Tax interns are expected to have general knowledge of financial statements, but they are not expected to know what to do right off the bat. Having an introductory knowledge of taxation beforehand will be beneficial, but is not mandatory. Professional communication and basic Excel skills are a must.