Before I entered college, I also worried about all that stuff. But halfway through, I just started doing whatever.... As long as you keep busy, have some clinical experience, some research, some idea as to why you want to be a doctor, I think you'll be fine.
As for the class thing, I think it is important to remember that every class that is hard to others is easy for someone and vice versa. So just take things that you are interested in. It doesn't really matter how hard or easy they are. So take upper level classes if you want to. You know how much you can handle better than me (some random person over the internet). I'd suggest spending maybe a month in college just taking classes to get used to it before deciding on extracurriculars.
Also, during my undergrad, I found that the higher the number, the easier the class cause the professors were better. It might help to ask previous people in your school who had taken the class before when deciding between a 200 and a 400.
Edit: Oh! and I found the so called "weeder" classes to be the easier than the upper level classes so I think if you are looking to join any extracurriculars freshman/sophomore year is a good time to start. I had to drop some things during/after junior year 🙁 . Just like layer them on slowly ((find one, wait a month or something, add another one, etc.)) and if you ever find yourself overloaded, you can always drop things!