Location matters a lot - you can have a great store, but not a lot of traffic if you don't have the right location. Close to a gym, close to where a lot of offices are that people want to be in shape, etc.
Need a good product mix that people want - more bodybuilding oriented if closer to a gym, but also a mix of vitamins and minerals for an older mixed crowd.
Gotta be friendly and give good advice, for people coming into the store, a lot of them like to know what is good, what works, and want to be treated nicely.
Supplements can be bought from various distributors. I work for DNA Sports Nutrition, which is a supplement distributor on the West Coast that specializes in sports nutrition, not really vitamins and minerals, etc.