I would prepare first and foremost by having a clear and well-written Student Curriculum Vitae as well as a cover letter. Bring these to your interview even though you likely already mailed or submitted them online. It is always good to have extra copies in case they ask for a hard copy at the interview. Career Services is a great place to go to edit and give feedback on these documents.
With that being said, make sure you know what is on your Vitae and can talk in more detail about things you included. You will likely be asked questions about your experiences.
I would recommend speaking from your experiences to answer the interview questions as much as possible. Tie in things you have done in volunteer or past work experiences to provide a foundation for your responses.
Go into the interview with a few standard questions prepared to ask your interviewers at the end. They will ask you if you have questions and it is always good to have something to say as well as take advantage of this to ask things you really do want to know (i.e., what your intern role will look like, grade levels you will work with, how supervision works, etc.)! It is also good to ask questions when appropriate during the interview if a good time presents itself. I think some of the best interviews are when they are more like a conversation.
Lastly, relax! It is normal to be nervous, but be confident in your experience. Confidence and the way you present yourself speak louder than words.