My advice would be...get a trainer. In my years of training and showing driving horses on the AMHA circut I have seen so many accidents happen because of green trainers/drivers (not saying you are, but just in general). A lot of people I know think that since there so small they aren't going to be hard to train or that they can't hurt you. That infact is NOT true. I younger gal I knew broke a 3 year old gelding. It was her first time breaking a horse, and I have to say she did a pretyy good job. Expect she really pushed him and skipped over a lot of the foundation training. He had only been in the shaftes for a month and she took him to his first show. When she was showing him in a judges command class, she asked him for a wheel pivot (which she had not taught him yet) and the horse kept resisting and started rearing, and she pulled back trying to get him to stop but ended up pulling him right onto her lap, breaking both shaftes on her $1700 cart! She's now working with me and her geldings doing really nice!
But just because driving is such a dangerous disipline (more dangerous than riding) I would highly suggest getting a trainer, just so something doesn't go wrong and so the horse gets a good foundation =]