If you want to be a trainer, get a job as soon as possible in a gym and start training as soon as you can. You want to see what it is really like now rather than later - you don't want to plan on being a trainer in a few years or worse yet go to school for it and find out you despise it. A lot of people like training themselves, but training people is as fun as you might think, especially come your 8th+ session of the day.
If you want to be a strength coach for a college you will either need to play D1 sports or plan on going to graduate school and being an assistant (which still isn't easy to land the gig). Big schools prefer former players to come into the strength and conditioning department when possible.
This is important because big schools are the only ones have the $$ to pay graduate assistants' tuition and perhaps give you a small stipend. Most other graduate students get aid by being teacher's assistants or helping out in the lab.. but if you are going to be an assistant strength coach / graduate aid you are not going to have time to do something like that.
Smaller schools don't have the budget to pay assistants, so you will have to volunteer while in graduate school while simultaneously paying for graduate school at most universities (i.e. ones not nationally known for having great sports teams). This means you will not only work your ass off to go to school and be a grad assistant, but you will also be going into serious debt at the same time.
Of course, you could always start your own business and try to train athletes in the off-season, but that is a LOT easier said than done.