Exelent choice, is good money, but it is a good question too
I think you will have to study Teorical knowledge of High Intensity Training, Advanced Nutrition, Anatomy, you will get a certification of the sports medicine asociation or any relevant certification, and you will learn some of the differents needs in the differents sports to know what each athletes need to develope.
By the other hand you will need your office in an exelent gym or your own gym, thats how i work.
Oh!!! i forget the experience you will get with the years, and have a good shape....
My best wishes.
My friend Bert Vila in Miami have his own gym, one Audi, one ferrari, one 2004 Corvette, a big house, he is training a big group of professional football players with Super Slow.