I love comeptetive trail rides, I did it for three years with my old pony. Every trail is different, we had some where we had to go up and down STEEP hills, and go through narrow trees with ribbons and all, some had where you have to pull your horse inbetween cones and back them up, and you had to trot or canter in water, and there were bridges you had to cross over.
To get your pony in shape, you should find hills, they don't have to be steep- but just enough, so over time your pony will be more than ready to go through the trail.
The judges judge two main things -- your horsemanship skills & how you handle, and take care of your horse during the different obstacles, and how your horse handles the obstacles.
Alot of people have in their mind that they HAVE to finish first, but infact everyone finishes at the same time, its not a race, racing would get you or your horse hurt. You have to give your horse breathing room, and allow your horse to walk and stretch some, with trots and canters too.
But other than that they are so much fun! You can relax and enjoy it. I'd buy a cantle cover so you can be a little comfy in your saddle it your seat is hard. Also you can find, esp. For an english saddle, you can find saddle bags and they usually go on the back of the cantle, you can store water bottles, and food.
As for your clothing, wear whatevers comfortable. And boots, I usually wear jeans, paddock boots, and half chaps and all. Just casual.