You should start shadowing when you have the time to do so without compromising your studying. Early in undergrad may be good for several reasons: you're taking introductory courses which may be less demanding than organic chemistry, you're not trying to study for the MCAT at the same time, and if you decide medicine isn't for you, you haven't spent all that time taking the pre-reqs.
Remember, you don't need to shadow a doc for your entire four years of undergrad. Spend a couple days - enough time for the doctor to get to know you, and enough time for you to understand what it is your physician does. Then start looking for other clinical/volunteering opportunities where you can make a contribution.
Best of luck,
-z