For starters, one of the things you can do is be a peer mentor or work for a Boys & Girls Club where students give advice and act as role models for younger students. Visit their Web site at www.bgca.org to find a local club and get more information. You can also join the Girl Scouts or Boy Scouts and become a counselor. These would be good ways to hone your skills and prepare yourself for business ownership.
You should find a mentor, someone who is already a success coach, who can guide you through the process of starting this type of business. A good mentor is invaluable in learning the skills needed not only for, say, success coaching, but also for successful business ownership--namely, the ins and outs of marketing, sales, pricing, image (as you've indicated, you don't want your age to be a deterrent to potential clients), accounting and so on.
Finally, there are coach training programs available--Coach U is one of the more well-known programs. Their Web site tells you all about their programs and can also help you find a coach (if you just want to talk to a few coaches before you jump in and start a business). Coach U even has teleclasses available where you can "attend class" over the phone.