What do you need to do to prepare for the exam? I'm honestly really nervous.
Hwahaha! I finally have experience with the JMSS entry exam and I can now bestow upon you this wisdom. 
I honestly expected the test to be super difficult, but it's actually pretty easy (imo).
Here are some tips to help you prepare:
• Practice answering with practice tests under timed conditions (science reasoning, numerical reasoning, mathematics) 
• Practice writing pracs and scientific essays under timed conditions (give yourself 5 minutes for planning and 25 minuts for writing
• Study year 9 math concepts and know them well because you need to complete the questions quickly (quadratics, trig, indices, algebra)
• Study year 9 and 8 science concepts (the test isn't that heavy with scientific content, if that makes sense) 
I personally didn't go to JAC, Henderson or Edu-Kingdom for their JMSS preparation classes. So you don't HAVE to go tutoring for this test, but I guess it helps with working under timed conditions. Apparently they make the tests way harder than they actually are, so I don't think it's worth going (up to you though). I didn't buy any of the practice tests either. Just practice working on answering the questions accurately and quickly. 
Good luck! :DD
If you have any more questions, you can ask me or anyone else on this thread!