I just finished Y9 - here are the things i wish i knew before I started:
teachers really do want to help you when you choose your GCSEs, even though it feels like they don't. They know your skills. try to trust them (but remember that at the end you have to study for 2 years)
similarly - i had trouble with my parents wanting t decide my GCSEs for me. I didn't choose what they wanted me to, and while it felt terrible at the time (there was lots of crying!) i am now so so so happy with the options i have chosen, and excited to start my courses. Lesson - your parents will not hate you forever, you eventually have to learn to be independent. take the courses you love and that you are interested in (being good helps too!)
third, it's time to ramp up the revision on core subjects!
following on, if you have end of year/end of topic exams, try harder than usual in the subjects you think you will be taking. get a feel for the pressure, and also whether your like for it can withstand that pressure
finally - never has it been so important to have good relationships with teachers! especially in subjects you will be taking further!
remember to have fun with your friends and family, but don't forget what is most important to YOU, whatever that is.
good luck! Y9 for me was a great year and I'm sad it's over x