I know someone that goes to pre-uni and I checked up on their work and what content they covered...so although it might not seem like its helping you, in the long run (for yr 10,11,12) it will provide with a head-start than other students, if you want to do science in those years. Anyways, if you want to excel in science and study for it, then look through the science textbooks for your grade in your school. In my case, the school used core science stage 5 and oxford science yr9.
The key to doing well in science, is to read, take notes and understand the key concepts. Also it is great to make your notes in your own words because there is no point in rewriting the textbook if you can't understand what the concept is. If you are doing extremely bad (like failing), then it would probably be a good choice to ask your teacher for help and teachers always like kids with questions. Also, try different study techniques because you have a lot of time to find your study pattern; some people read paragraphs and understand, while others require visuals or sounds (like listening to your notes, instead of reading them).
As for getting a tutor, if you are good at studying by yourself, then there is no point. Tutors don't do much...all they do is set work and answer your questions. Personally, I can't learn from tutors, but it depends on person to person, so if pre-uni hasn't worked, just try your teachers and self-study and maybe that is the way for you to go! Also, try different study techniques because you have a lot of time to find your study pattern!
Hope this helps!