My advice ...
1. Dive as often as you can ... 50 dives is just about enough to begin to suspect how much you don't know about diving. Get your instructor's certification, but do it in a way that helps you develop a reputation as a competent instructor.
2. See if you can get a job in a dive shop ... even if it's just doing menial work initially. Use it to learn as much as you can about the business, and to develop as many set of skills as you can related to dive shop operation ... things like regulator servicing, equipment repair, small engine/compressor servicing, operating a boat, and any number of other skills required for a successful dive operation. The more you know, the better your decisions will be when you reach a point where you can invest in owning your own business.
3. Develop some business skills outside of diving ... inventory management, book keeping, customer service, online sales, personnel management ... these are all skills that can make or break any small business.
4. Do some homework finding out why some dive shops succeed while others fail. There are many reasons ... most have to do with customer service, reputation, and developing a business plan that tracks with how customers like to shop. Develop your business plan accordingly.
5. Don't go into the business because you love to dive ... go into it because you love running a dive shop. Otherwise the diving will start to feel like work and before you know it you won't love to dive anymore.
6. Only invest as much into any business as you can afford to lose ... not that you plan to lose, but if you do it won't be an irrecoverable loss.
It's good to have dreams ... and at your age this is a good dream to have. But running a successful business is hard work. The hours can be long and the risks can be stressful ... and the rewards can be very much awesome-ass if you're successful. The difference between success and failure usually boils down to how much effort you've put into learning the business before you lay your money down and hang your name on the front door ...
... Bob (Grateful Diver)