Don't stress out! :)Right now all you need to do is successfully transition to high school. Don't worry about AP classes right now. You probably won't be allowed to take them until at least next year. That is the policy at most schools. However, you should ask your counselor. It is his/her job to help you understand what classes you should take and explain the school's policies about AP classes.
You shouldn't take AP classes until you feel ready for them. Talk to your counselor, teachers, and older students. They will have useful advice to help you decide what classes are best for you and when you should take them. It is not good to take AP classes too early, because taking an AP class doesn't really have a point if you don't do well in it. Generally speaking, I'd say that one or two AP classes in sophomore year, at most, are a good star. Plenty of people don't start AP classes until junior year. But it all depends on what you personally can handle. Again, talk to your counselor, teachers, and older students at your school. Also, when you do start signing up for AP classes (which shouldn't be for a while!), remember to play to your strengths. You don';t have to take every AP class available. An A in honors English is better than a D in AP Literature!
In high school, most advanced classes have summer work. In fact, plenty of standard high school classes, though usually only English, have summer work too. It will not be hard if you don't procrastinate!
As for your electives, you are on the right path. Choose classes that are interesting to you and help you explore what you want to do in the future as well as prepare for it. Again, talk to your teachers and fellow students! They will be a big help. Science classes are definitely important if you want to be in the medical field, but don't feel pressured to take EVERY science class. Also, don't take too many difficult electives. Your required classes should come first. It also not necessarily a bad idea to take one "relaxing" elective when you have an otherwise difficult set of classes.
It's great that you are already thinking about things like this. It shows that you are a very focused and driven student. However, don't stress out! Like I said before, freshman year is a time to transition to high school. It's a time to get used to a very different place, so don't overload yourself, and try to have fun too!
I just graduated high school. :)