I'm sorry, I must have misread, but I thought you said you wanted to start doing it for fun.
I still stand by what I said; Go to clinics, get lessons, talk to trainers, talk to riders, read books, watch dvd's, anything you can get your hands on. Ride as many different horses as you can, try out as many different disciplines as you can. You need a multitiude of experience of everything you can think of to become a trainer.
Try to find a trainer/instructor you can work with, but I do know this is a lot harder to do in AUS than in the US, as I am currently trying to find a trainer to work with and it is not easy!
I think the biggest step to becoming trainer may be realising and acknowledging how much you don't know; And how much there is always still to learn. You will learn things from every single horse and every single rider.