Do a regular resume, but add a section titled CYCLING SKILLS AND EXPERIENCES. In that section list:
1. the bike(s) you own
2. your riding frequency (days per week/month you're on your bike)
3. milage per week/month
4. types of riding you do (road, mountain, cyclocross...)
5. the types of maintenance that you have performed on your bikes

Remember that at 16 yo. you won't be expected to have a great resume but you can make your bicycling skills and abilities your strength.

The real key is to make sure you have the name of the person you spoke to and write a cover letter to accompany the resume explaining that you were excited to hear from _____ that summer help was going to be taken on and you would be honored to be considered for one of the positions. You could also add that as a frequent visitor to the shop, you have always been impressed with their professionalism and customer service and would be pleased to work and learn in that kind of environment.

Good luck!!!