In order to be successful in AP Environmental Science (APES), you will need to have a strong grasp on biology and chemistry at a minimum. You do not "need" to be familiar with physics in order to be successful in APES, but if you are familiar with physics as well you will have a much easier time understanding some of the concepts involved in the course. It is also necessary that you feel comfortable with algebra and especially dimensional analysis from a math standpoint.
Concepts in APES are not necessarily "harder" than you may be used to in a typical science class, but the sheer amount of concepts and pace that they must be learned at makes the course a challenge for many students. Because of the amount of information involved with APES and the background knowledge necessary to be successful, you should already have chapter 2 from the textbook read and objectives completed even before the course begins. You will "hit the ground running" from day one! As the year goes on, you should expect approximately two hours of work outside of class for every one hour of work during class. This is a college level course that, again, covers a lot of material!
The first new concept that you will learn about in class is that of sustainability. Most students have an idea of what they think this means, but the concept of sustainability entails quite a bit more than what you may be thinking. Wisconsin Public Television put out a great talk on sustainability by Paul Robbins who is the director of the Nelson Institute for Environmental Studies. It is highly recommended that you watch this video as an AP Environmental Science (APES) student. The video may be found at the link below.
The following video clip (~1 minute) is simply meant to get you thinking about simple changes you can make towards being more environmentally conscious. Think about how effective/ineffective the messaging is as you watch.