As a student who is going into DP Year 2 in a week, I can tell you that it is best to relax. Pre-ib is going to be a chill experience and there isn't really anything you should prep for. Just enjoy G9 & 10 and the time that you have before you really need to work in the DP. If you really want to look at some material, then I would recommend you to look at some basic Algebra, quadratics, exponents, etc for math and if you are in extended maths, try to do some harder problems whenever you learn something new. For the language courses that you take, learn some vocabulary beforehand and challenge yourself a little before returning to school (like Spanish, French, etc).
I don't know if you get a syllabus before G9 or if you get one at all, but try to look for typical coursework, for that year, online and try to prep/do the first few days of work (before doing it in class). This works really well if you have certain materials or textbooks that you use for class because you can always read a few chapters ahead and do a day's/2 days worth of work beforehand. If you want to do well, PAY ATTENTION! Be respectful in class and be nice to your teacher, they are your best bet for improving in anything (unless they are absolute dickheads and don't care about their students/subject).
Generally, just try to participate in class and yes, it is tempting to joke around with friends but keep that to a minimum. Be as concise, clear and organized as you can be, stay slightly ahead, and meet your deadlines. That's basically the most important stuff. If you do that and hand everything in on time, teachers will know that you are hardworking and reliable, and it already gives you a few bonus points with grades (this is what really helps you in DP)
Ps. try to do homework as much as you can. I know that there is a lot to do if 8 teachers give you half an hour of work every day, but at least finish some of the work or do it quickly. Mostly, homework is to help you learn and understand the topic, so just try to do it. And remember that you don't have to be perfect. Enjoy the MYP and good luck!