Spoiler alert: the track will totally ruin street riding for you. That being said, you should definitely do it, even if you decide afterwards it's not your thing.
As far as gear goes, get in touch with your local track day provider, and find out what they require. I ride with: helmet, boots, gloves, 1-piece suit, back protector, and armored shorts. Good gear can be purchased used, and it's a very worthwhile investment. Two-piece suits are fine, if you've got one; no need to go out and buy a 1-piece suit for your first track day.
As far as bike preparation goes, again, get in touch with your local provider to find out what they require. Modern sport bikes are so capable, chances are you won't need to do much. The usual list is making sure you've got plenty of tire and brake pad material left, clean & lube your chain (add a little slack too), and unplug or tape up your lights. You might need to replace coolant with water + water wetter.
Depending on who you ride with, there may be some vendors at your track day. If there is a suspension guru, go to him. Pay whatever it costs, and get your suspension set up for you. Suspension set up is a black art, and the people who can do it are worth their weight in gold. If there is a tire vendor, find out what tire pressures you should be running and set them. If there are instructors, and you can get some time with them, DO IT. Ask them to follow you and critique. Talk to them about lines, body positioning, and how to be more smooth.
Do not worry about being the slow guy. Everyone is there to learn, and so are you. It's a track day --- there are no points or trophies, and everyone's got to go back to work on Monday. Like others have said, it is 100% the responsibility of the faster rider to get around you. The best thing you can do is focus on your own riding, and be predictable and smooth.
Unfortunately, this is an expensive hobby. Track time and tires are basically non-negotiable, and they cost a lot. That being said, you can find out ways to save money, e.g., buying take-offs, camping at the track, bringing your own food, splitting travel costs with friends, etc. The single biggest way to save money is to not crash. Crashing hurts, and it's very expensive. Don't do it. This basically means not riding above your head.
It seems like a lot to process, and it kind of is. Nothing can really prepare you for your first session on a hot track, and that's okay. It's amazing, bottom line. Seriously, do it as soon as you can.