The shirt can be plain, but I prefer a patterned shirt because it hides any dirt spots you might get on it. I do all my own grooming, tacking, etc. on my horse and inevitably, I get some dirt on my shirt during the day. For a clean, "put-together" look, coordinate your shirt and saddle blanket. Regarding the pants, always keep them untucked for classes and make sure they are long enough. The pants should be at the soles of your boots when you are seated in the saddle. You can keep them rolled up or tucked in before class to keep them cleaner, but always untucked for class. Most shows are fine with you wearing a helmet rather than a cowboy hat. Some shows require helmets if you are under the age of 18. Check the show bill or ask the show organizers what they require for head gear.