If poss walk the courses day before, gives you time to think about them, and avoids rushing around on the morning of the day. And have fun, do one and you'll be smitten to do others!! Which one are u doing?
The only thing I would say is if it was me id arrive early to work out where all the areans are for all 3 disiplins, and id make sure I was warmed up all in good time for each event,
Keep a cool head, try not to expect to much and enjoy your self. and more importantly if this is all new for you horse try to let them enjoy it as much as you do.
Are you doing it on your own or are you with someone else? I did my first ODE a few weeks ago and went with my friend. She has done quite a bit of BE and she was taking her youngster in the mini ODE that I was doing. We had a great time and it was brill having someone there who knew what to do!
We're doing another one on the Bank Holiday Monday