The first thing you should do is find a show in your area and go and watch. Big, little, 4H, doesn't matter. What classes look interesting, challenging and are something you and your horse can do? What are people wearing and how are the horses prepared and presented? You will learn alot more sitting in the bleachers watching than you will ever in a book. Offer to help be part of the ring crew for a 4H show.
It can be very demoralizing to show up thinking you're ready only to be clueless in what's going on. Some judges can be brutal and will crush you as well. Understand the terminology and class descriptions.
Local groups will advertise at feed and tack stores. They might show up on CL. If you have a state or county horse group, join it. Typically this time of year, everyone has schooling shows. Horses will be fury and slightly out of shape. It's just an opportunity for people to get back out, maybe show a young horse for the first time. Very informal. Usually no show clothes (chaps or sparkles). Fun shows are another good place to start. They will have some traditional classes but might also have some "fun" classes. They might be timed events, eliminations (command class or bareback dollarbill), WT or costume. Horses will not be turned out perfectly, show clothes are left at home, classes are cheap and the expectations are not too high. Most first timers start with halter and showmanship classes. It's funny because those two are probably the 2 hardest to do right and win but they're a good safe way to break the ice.