Your best option is to work under a well known trainer to build your experience and go from there.
I agree on that. Personally I could not care less about "certification" or "schools for horse training", the most knowledgeable horse people and trainers I know are tought by their horses and themself and so are the most famous horse trainers too!
When I came to Canada 4 years ago I volunteered for at least 6 months at a place where I liked the trainer A LOT and learned so much about actually training a horse from zero than I ever had in years of lessons and horse ownership in Germany and show jumping across Europe...
By now this amazing horse trainer is my boyfriend and we expanded the business together and it is a great lifestyle, but you have to be in it with your heart because you usually will not get rich with it