First things first, you need to read all ur rules that will state all the equipement ur allowed to use.
Definetly check out what ur horse is like out in a gallop (maybe at the beach or bush or forrest or trails) and see what ur control is like
If you don't wanna spend a fortune on boots get some splint boots to protect the legs
In australia in the pony club events it's a must to wear long sleeves on cross country. But in EFA(equestine federation of australia) it's not necessary to wear long sleeves.
I find the pony club rules are alot now strict so you might wanna check that out.
On xc ALWAYS carry a crop if allowed and ur horse isn't scared of them.
Also if allowed (donno where you are) wear a watch that counts ur time and set it 1 sec before you take off and use that as a guid to know if ur going slow or too fast, just remember to stop it at finsh flags or at least look at it (i forget if I get refusals)
CHAPSTICK!! Make sure you use chapstick on ur lips before doing cross country, u'll be glad you did