That is an extremely broad question...
I recommend finding someone who has produced safe, sane, and successful games horses and enlist their help and advice on-site. Training for any discipline well involves a lot more than just getting on and practicing what the finished product is. There's foundation to be laid, and if that's gotten wrong, then there's a lot of tough fixing to do.
Find a trainer, or at the very least an experienced games rider who has brought a greenie along and understands and is able to accomplish the task properly.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown