Hey there! Showing should be a fun experience so don't get too nervous about it! Just get out there and do your best. I went to a local hunter show last year and I wore a navy blue show jacket, a white show shirt with khaki breeches (beige is also acceptable). I had on tall boots and my hair was kept up in a hair net under my IRH helmet.
For the helmet, I've seen people who have just put a velvet helmet cover over their schooling helmet. Also, if you're a younger rider, you'd generally wear jodhpurs and garter straps with paddock boots instead of breeches and tall boots.
Hope this helps! Best of luck with showing!