OP since you recently posted about struggling with your grades right now, I'd focus just on you figuring out how to manage your coursework before worrying about ECs. Your undergraduate GPA will be one of the hardest parts of your application to change later. So, getting your academic performance on track should be your top priority. It's MUCH easier to take time to increase your EC involvement later on than it is to make up for poor academic performance. Personally, I was minimally involved in ECs until about half way through my sophomore year when I felt comfortable with my coursework.
Also, remember that once you start working on your ECs you don't have to do everything at once. You should aim for long-term involvements in 2-3 service commitments where you eventually take on a leadership role. Even 1 hour a week over a 3-4 year period will be good--steady, long term-commitment is what counts. You can TA and tutor for one or two semesters--you don't have to keep it up all the time. Winter break and spring break are great times to get some shadowing done, perhaps close to home with a primary care physician. Research can be done over your summers with a reduced commitment during the school year.