Well, you all will probably think I'm crazy, but the only way I was able to get to my family doc to ask him if I could shadow was to make an appt! It worked and he was very cool about it. I would have done the same for shadowing the DO I also see in this practice (this guy was an MD) but I prefered shadowing a DO I work with who is NOT a primary care doc. (But the reason I wanted to shadow him was partly because I have worked with him and happen to know he is a wonderful person, and partly because he is an adjunct faculty at my 1st choice med school!)
However, that strategy may not work if you have never seen a doctor before, but if you have ever gone to a doctor for anything - and I'm sure everyone has! - Call the office and see if there's a DO in the group. Chances are there is at least one, and even if you haven't had the chance to see that particular doc before, having gone to the group itself gives you some credibility. It may cost you (or your parents!) a copay, but it's worth it.
Also, similar to finding work where there are plenty of docs, you can check around and see if there is a free clinic in your town. That is a great way to meet docs and get your volunteer/community service time in all in one fell swoop!
Good luck!