Since astrobiology is more interdisciplinary field, can i take these tests in varied subjects(physics, chemistry,) along with biology?
If you're mainly interested in biology, then it won't be necessary to take those tests either. The AP test is hard enough to prepare for, let alone chemistry and physics. In fact, it would take a whole summer (and some) to really begin understanding this material.
I'd begin with a trip to the library. Check out a grade-12 textbook on biology and start reading it. This will provide you with a solid foundation of the subject matter. Take notes, highlight important concepts, and try to understand the main ideas and connections between different topics. Also, use our forum to ask questions that you find at the end of each chapter. There are also plenty of practice tests online (I can help you find those once you're ready); that's what I used to do to prepare for my university exams. I think the key to understanding biological concepts is by watching animations;
this website is a treasure trove of great animations on a wide array of concepts (they also have a YouTube channel if you prefer). Remember that learning AP biology is a gradual process, so be patient with yourself and give yourself enough time to grasp the concepts.