So fun! Welcome!
Read everything you can get your hands on - but a caveat on EN. It does a ton of marketing and may make you think you need a lot more specialized “stuff” than you actually do. I’d start instead with http://www.useventing.com/ - I think there are even some articles geared toward doing your first event.
Strongly suggest you scope out the event you plan to do so you can get the lay of the land and see how things are run, whether people braid, how far between parking and XC start, whether there’s food, etc. Walk every course you can get to, and then watch it ridden. So if the facility runs more than one event per season, think about entering the second one and just watching at the first. I like to watch a few riders at every jump - you can get a sense of how it jumps and what works (and doesn’t!). If you know the facility and how things run, you’ll be so much more comfortable when there with your horse. Also, if you can go school there before competing, that’s also a great idea.
See if you can borrow a body protector. As for boots, meh. I don’t use them at BN, but most people around here do.
The rules aren’t really a lot to get through - just hunker down. It’s winter, after all. 