OK, so based on that information, I would say the things you need to work on most are math, and being able to write a 5 paragraph essay, because you will have to do that in both English and History. Study skills might also be something you need to work on, but lots of high schoolers still have issues with that-just stay on top of homework/projects/studying so you don't wind up with an impossible amount of work at the last minute.
The nice thing is that Science and History come around to the same concepts over and over again, so while others in your class might have more background knowledge from the last time they learned it in school, if you can learn it and remember it you will be fine.
For math, try using khanacademy.org this summer-you can start with the 6th grade math and move through it at your own pace. There are videos to teach all the concepts and practice questions to answer until you master each topic-it's really great and completely free. If you are dedicated, I will bet you can get through both 6th and 7th grade Math this summer. Writing might be a little harder, but for that I would try the videos and lessons on learnzillion.com (you can watch math videos there too, but kahnacademy is definitely better for math.
Finally, definitely get in and meet with a guidance counselor because they can be a really great asset and try to set up a time to talk to your teachers about where you are coming from and how you are really motivated to do well and they should all be more than willing to help you too.