Rule number 1: Set a budget and stick to it by starting with the basics and adding on as you can afford.
Rule number 2: See rule #1.
Begin with your show bill. Pick one type of class for your first show: English, Western, Dressage, etc. We are going to accessorize our Sorrel Paint Mare so we will start with Western Attire. We'll start by purchasing a show shirt that color coordinates with our horse and is appropriate for the class type we plan to enter, then we can use the basics we already own, can borrow, or can find at goodwill to the extent possible.