My advice to you would be to start now, well sort of..... I will start by saying that many people will tell you that you will probably fail, and that is certainly a possibility. That is not to say that you cant be successful either. I would suggest you create a website selling scuba equipment and/or buy and sell scuba related items via ebay or craigslist. This will give you some real world experience in dealing with customers, keeping them happy, and working with your partner. I am sure one of your parents will create a paypal account for your new venture.
College has certainly been pushed onto society as being the way to "make it" in America but certainly isn't necessary in many cases. Buying and selling scuba gear yourself, creating a webstore that utilizes drop shipping and interacting with customers will give you more business knowledge than a business professor that is a professor rather than running a business, lol. assuming you go to a decent school, you will have the added bonus of having tens of thousands of dollars worth of school debt hovering over your head while you are trying to start and run your business.
Prior to doing any of that, try to get work at a dive shop, volunteer if you have to if you aren't old enough to legally work yet. This will give you an idea of how business run their day to day operations. I doubt you will be given the financial books to study, but you can get an idea of what people are buying and what items the shop is pushing (this could be a clue to the more profitable items in the shop). Ask tons of questions, learn as much as you can, and apply yourself and you can succeed.
Seriously reconsider the whole partnership idea. People are hard to work with over long periods of time. You cant consider every situation that you may face in your new venture, but the chances of you two agreeing on everything are slim to none. No one in this world will take better care of you than yourself. It is easier to do things your way than have to consult your partner for every decision you will have to make in the future.
Set some small goals for yourself and accomplish them. Start out with your Christmas money and buy and sell a few things, find deals on stuff and "flip them" hopefully for a profit. If you are successful, by the time you are 18 you could have enough start up change to open your shop. Understand that not everything is going to happen like you hope it would, you will make mistakes, you will lose money, people will piss you off, its all part of running a business. Try to learn from these bumps in the road and move forward from them.
Use the opinions of those that believe you will fail as motivation. Don't waste your time trying to convince them otherwise, show them with action and results. This is America, you still have some freedoms and abilities to succeed left for now. If you want it bad enough, you can make it happen.
good luck!