helpmepls I haven't done the John Monash test, but I could try and provide as much information as possible!
Firstly, you'll want to see what you'll be tested on. I pretty sure your tested on Mathematical skills, Science Interest and communications and science experiment/procedure (Might be wrong on this though, feel free to correct me). You should probably go to a tutor but I'm not saying you have to. It would help you understand the content better. In terms of tricks, try cherry-picking (which is choosing the easier questions to do first) which will greatly help with saving time. You're in year 8 so the test may seem far away but before you know it, the test will be closer than ever. Always make use of your time whether it's doing practise tests or simply learning new content. I'd start by checking out the John Monash Science School Website for more information! That's all I really have, as I said, I've never done this test but I hope I managed to help you in some way! Good luck!