What WE did when we first jumped in, was attend a couple of shows first. While there we picked people's brains who were obviously bigger farms and some trainers...... just don't do it when they have a horse being prepared to go into the show ring. And take note of what they are wearing, plus what show tack is being used. There are usually vendors there for both show tack and show clothes, unless it's a really TINY show. But, check them out.
Do you have an AMHR club in your general area? Are you a member? If not, become one. Other members will be helpful with your questions. Start showing at your closest location(s) and see how it goes. Your horse will have to earn points to qualify for the nationals.
Yes, your horse will need to be fully body clipped, so invest in blanket(s). We would do a rough clip about 4 to 6 weeks prior to the first show of the season, then do another one about 2 weeks prior, and a day prior to leaving for the show we'd do the last bath and a final, detailed clip......Once at the show, we'd do some more detail work on the horse's face/head, in preparation for doing the make-up. We rarely bathed the horse(s) AT the show grounds, but others did.
Be prepared to invest in a list of show supplies..........