I love IEA, I'm really glad you decided to join. You basically need everything you would need to show in.
SEI, ASTM certified helmet(I don't like having the dial ones for showing but doesn't matter for IEA atleast), Navy(I prefer navy), black, or hunter green jacket(not sure if hunter green is allowed in all zones though so pls make sure), white show shirt with collar and preferably long sleeve, tan breeches, black tall boots, black gloves and all the other things are just accessories.
Just make sure you look your best and have fun!
Navy isn't required in IEA in my zone but is preferred for equitation :)
when i did iea we were required to wear a navy show coat but i'm not sure if that's the same for every zone.. i also just wore a white show shirt, tan breaches, a black leather belt, and polished tall boots! good luck and have fun!!