I would say that one place to start would be at your local Missouri Fox Trotting Horse Breed Association (MFTHBA). They should be able to provide some guidance on getting started. You could attend some of their show events and start asking around. All Show Members had to get started somehow, so they should be stocked with information to help you. There is an annual Fox Trotter Show in Ava, Missouri called the "Celebration", which is a really big show. But you could get a list of all the shows for the year and attend as many of them as possible.
You might also consider getting some books on "Horse Show Judging for Beginners" or similar books. Understanding what a judge is looking for can help you develop both yours and your horses skills. You might find some related books in the "General Store" here. I took a look and there are over 164 books related to "Horse Shows".
Good Luck! Please share back as you move forward with your endeavours. I would love to know how it goes for you, what you find out, and where you get your best information.