I did my first western show 2 months after I started getting professional lessons. I rode western already on my farm, but I decided I wanted to start showing. Just be prepared for anything at shows. Get your horse used to the show atmosphere would be my best tip.
I trained a lot for my first local show. I rode or practiced showmanship everyday. I wanted everything to be perfect but it was a disaster. The mare I was showing spooked at a noise in the arena, and started running around me and rearing.
But don't let that put you off. Just train a lot, make sure you are competent cause its easy to over-estimate your skill level and have fun.