Because eventing has three disciplines, make sure you master all of them. Also, make sure you know how all disciplines work like the rules and how the points work. I have only done a little bit of eventing so I can't give you much advice but you should have a go! :)
Get in and have a go! First off make sure you read the rules that apply to you, to make sure all your gear is complying with the current standards and to find out what else has to be done before you can compete. Familiarise yourself with the layout of the competition to ensure you have enough time to get ready and warm up before each phase. Practice on cross country courses (if possible) and in a dressage arena before the show and make sure to put your horse over various fill e.g. boards. But mostly you just have to jump right in, everyone is supportive and where all in your position at one stage.
Good luck and have fun!!