The road to becoming a SCUBA Instructor is frought with decisions: Which diving society are you planning to teach for? There are several out there, and you'll want to find one you're comfortable with; one with which you can get involved, and see it through to your goal of becoming an instructor.
As far as the "academic" requirements you asked about, suffice to say that you'll need a good background in Physics, Physiology, Dive Computers and the RDP (Wheel and Table), Equipment, and subjects like that. Most dive societies will accept you into a Divemaster or equivalent program once you turn eighteen years of age. This requirement is due to the need for you to be insured, which is absolutely mandatory if you're teaching for any real dive society.
Once you've turned eighteen, done the diving, watermanship skills, presentation training, and other academics to become a Divemaster or equivalent, you can then find an Instructor program you like the looks of, and pursue it to your heart's content. This may or may not involve an interim qualification between the Divemaster and Instructor levels; be ready for whatever may be required of you, and work on developing patience as a virtue. You'll surely need it once you start teaching, as students are all individuals, and patience goes a long way toward your successful teaching goals.
As a new open water student, my recommendation would be to do all the diving you possibly can. There is no substitute for experience. Buoyancy is the single most important skill to master, with about 75 others running a close second. Whatever the next level of diver education is in your area, that's the one I'd concentrate on next. The diving industry is absolutely polluted with folks who think they should be an instructor by their fifth open water dive, so don't be one of those, and we'll all thank you.
You'll also need training in First Aid, Emergency Care, CPR, and Oxygen Provider skills in order to be a qualified Instructor. If you lack expertise in any of these areas, you cannot be a qualified, competent instructor. You could sign up for CPR at the dive shop, or perhaps at the local YMCA, or similar institution. Maybe after you advance a little in your diving levels, they'll have a Rescue Diver course for you. A qualified instructor can conduct a rescue without second thought. After all, when people put their lives in your hands, you'll want to be prepared for anything and everything that could happen.
Another thing I can suggest is dedication, although this is a "skill" which cannot be taught. It's inherent in the qualified instructor; it's something you either have on your own, or you don't. Of the jillions of persons who become instructors each year, only a FRACTION of them will ever teach any type of class whatsoever -- in their entire lifetime. Most are sitting around six months later wondering why they went to an Instructor program. Another great portion of them will teach one class every six months to two years, enough to keep the rating and pay for a year's worth of insurance.
If you're going to go the distance and become one of us who do this thing day in, day out, rain or shine, cold or hot, or whatever else we face, it'll take some planning and perseverance on your part. Becoming an Instructor is NOT a natural result of becoming a DIVER. Some folks make great divers, but when it comes to teaching, they're "out of the water." The instructor is a TEACHER, not a "Hollywood Hotshot," as we too often see in the diving world today. This is not a fashion show, a numbers game, or anything else like that; it's about instilling in others the opportunity to see the underwater realm in a confident and safe manner. We do this for the love of it, for the love of diving, and for the excited smiles of those we've turned on to it.
Learn everything you can about diving, dive travel, dive retailing, and dive locations. If you want to travel, don't need much money to make a living, and have unlimited time, you can see a lot of this world by working in the dive industry. There are plenty of openings for young persons who would like to travel and make a little spending money as well. Look across the internet to see where others are working, what they're doing in the dive world; draw from their experience and learn what you can from them.
Take every specialty course you find interesting. You don't have to be an expert at everything offered out there, but it won't hurt to be an expert at "something." As you progress through your diving career, you'll eventually light upon one or more areas which have a strong allure for you personally, and it'll be in these areas where you'll want to concentrate most. We all do especially well in areas of great personal interest.
Develop communication skills, good grammar and spelling, good writing skills and a sense for the individual needs of diverse groups of people. Know that every person out there who wishes to dive has a motivation for doing so. Be empathetic to that reason, that person, his or her needs, and learn how to make each of them feel as though they're the single most important person in the world, if just for one day.
Good luck, young person. I wish you all the best.
PADI MSDT- 178767