This isn't just for people who haven't gone to hackathons before, but also for people who went to one and had an underwhelming experience. Hackathons are all different, so don't let one bad experience shy you away from them.
So you're going to a hackathon. You've signed up, and you don't know quite what it is, but you're interested, maybe because you've heard good things about them or they sounded like fun. They're all pretty similar in concept, but when I think of hackathons, I think of coroporate hackathons and collegiate hackathons.
Corporate hackathons are generally smaller than collegiate hackathons. Both in size, and in time spent working. That's not to say they're bad, some of the ones I've been to have been better than some of the collegiate hackathons that I've been to, despite their size. These hackathons will generally be 50 people or less, and all the ones I've been to have been less than 24 hours, and they've also had participants go home for the night and come back in the morning. They also usually have no problem with providing enough food and snacks.
They're also much different from collegiate events in that professionals are allowed to these events, so you'll most likely end up working with people from companies with local branches. If you're around the Dallas area, the AT&T Foundry usually hosts a few of these every year. There are also larger corporate hackathons, which I can't say too much about, as I haven't been to more than a couple with much more than 50 people.
Collegiate hackathons usually only allow current students to participate in them. If you're in high school, they might require you to be above 18 as well. They usually give out more swag (read: goodies, so be prepared to carry lots back), and are generally either 24 hours or 48 hours. If you want to sleep, you'll generally sleep on the floor. They usually have lots of sponsors, lots of prizes, and generally are much larger events overall.
Another difference is that while corporate hackathons usually have a theme (IoT, mobile apps, big data, etc.), collegiate hackathons usually don't. They're also almost always fully organized by college students, which is pretty impressive considering the huge amount of money that goes into them.
The rest of this post is about collegiate hackathons, but some of it can be applied to corporate hackathons as well.
I'm a hardware hacker, so I usually bring my own hardware, which I've written about more over here. I'll also bring a day's worth of clothes, my phone, wallet, device chargers, plane tickets, headphones (nice for listening on long flights), deodorant, shampoo, and toothpaste/toothbrush, all in one duffel bag. I'll also bring a backpack with my hardware kit, laptop, and free space for free stuff. Feel free to bring resumes, as well, recruiters are there for a reason. It's also a good idea to bring a blanket, or clothes warm enough comfortably to sleep in. I usually go with jeans/sweatpants and a hoodie. (Side note: bring and use those toiletries. After a couple days without them you're going to reek and that's no fun. Also, shoutout to hackathons with showers, y'all are the best)
So, from the start, book your plane tickets early, arrive early (get there early and you get good spots), and make sure you have all your stuff packed the night before. It also helps to have a team and idea of what you're going to work on before you get to the event. If you don't, that's fine too, there are almost always team-building events beforehand to help people who don't have teammates connect and form a team.
Talk to the recruiters, don't be afraid to ask questions or take the free stuff they're giving out, a lot of the goodies they give out will be gone quickly. If you're interested in a company, you can take time to talk to their recruiter, and give out a copy of your resume. For large hackathons I usually spend almost an hour at the beginning for just going through the booths, looking around, asking questions, and talking. Then, after the opening ceremony, it's time to start working on your idea. At most collegiate hackathons, hardware checkout will open after the opening ceremony. If you want a big-ticket item to borrow from the hardware checkout, it's best to get there early before it gets taken.
While working on your hack, be sure to grab snacks and take some short breaks. Hackathons are usually very well stocked as far as snacks go, and they'll often have events like smash tournaments, either set up by the hackathon organizers themselves or sponsoring companies. And while some people like to brag about how much sleep they didn't get, getting a few hours of sleep can definitely help refresh you and give you a new mindset when tackling coding errors. On that note, don't be afraid to ask for mentors if you're having trouble.
In the final few hours before presenting, you'll usually want to make sure that you upload your submission to whatever website the organizers are using, as well as make some finishing touches. You'll usually be presenting expo-style, where you have an assigned table and judges come and walk through and look at everyone's project like they would at a science fair. You'll also usually have to pack up all of your stuff before you go to your table, and after presenting, head to the closing ceremony.
That's all great, but where do you find out about hackathons? Look no further, hackalist is my personal favorite for finding out about new hackathons I mean, you can sort by travel reimbursement, and it has them in order by month. This really only includes collegiate hackathons, though. If you want to find out about corporate hackathons, the best way I've found is by looking through EventBrite, asking around, or Googling.