The course has a reputation for being difficult, but it does depends on which year you are in certainly. I am an engineering major and took it my senior year as an elective. By that point in my education, it was actually not a big deal for me; however, it certainly was for many juniors taking the class. If I took it a year previous, I would have found it just as difficult I imagine.
As stated, your math courses will have presumably exposed you to all the fundamentals needed to approach the topic. This class will help cement these tools in your understanding, and give you real practice in using them.
You could confer with textbooks regarding vector calculus, and so forth, but ultimately you will spend a good deal of time easing into the topic once you start the course. If you were to procure a copy of the text, you would notice that the first entire chapter is about vector calculus review. Your class should begin with such a 'review,' and lead you to the material in a relatively comfortable fashion. No worries, and you will enjoy the text. Griffith's is a friendly author, who even speaks in first person, it is almost like he is leading you by the hand through the material. I took Jackson electrodynamics (grad level) last year, and am studying for my PhD qualifying examinations now, but I studied from Griffiths, not Jackson when reviewing electrodynamics. What I mean to say is, it is a good book. One to hold onto for your reference. There is nothing really you have to prepare for given that you will learn it as you go, but the other poster's topics list is good and should be all you need. I also agree that the best preparation is mental preparation. Best of luck!