Hidaaa Hi There! I didn't try out for JMSS but I could try and give some tips. Maybe go to the "Ask me questions about the JMSS Test and Interview Process" post made by kyroh. It should help with some stuff.
In terms of the test, tutoring with JAC should help a lot > asianasen just go to jac tutoring for jmss and you will be fine.
I'd suggest fully studying and making sure you understand the yr 9 curriculum of math and science.
SpaceLover but main advice i can tell you is fully study yr 9 curriculum of math and science.
If you still feel stuck, let me know and I'll assist you the best I can!
However, this only 'scratches the surface' of things.
In terms of the interview there's two interviews, a group and the 1 on 1. For group, taking initiative and suggesting new ideas is strongly recommended. However, it only accounts for 30% of the interview application so if you do bad, it's not the end!
The other 70% is the 1 on 1 interview. You have to find a balance between confidence and boasting. Somewhere between the middle is good! Talking about your love for science will also catch their attention! Also use eye contact! I got this information from a past post from SpaceLover so shoutout to them! Another Shoutout to @asianasen for where to go to tutor for the JMSS Test!
I wish you good luck for your studies!