I am a bit confused about your question. Do you mean before a competition or before a training lesson ?
Before a competition, I first get myself prepared. I get dressed in my gear and my mom helps me do my hair. I watch a test on the video camera, to a have a clear view of what test I will be riding and how to improve it. I then go to my horse's stable. I like to be around my horse because it cools my nerves a bit hahah. I would normally start by grooming my horse and braiding his main. I then tack him up and get on. While I'm walking him out, before I start warming up, I replay my test in my head several times to make sure I know it well. Once I start my training I am completely focused on my trainer and my horse.
Before a lesson, I groom my horse and then tack him up. I get on 15 minutes before my lesson starts, to make sure that I have walked my horse enough. I then do a light warm up to get the muscles ready for the exercises during my lesson.
Thanks for your question,