Most of this is going to sound basic to the point of comedy, but:
-Enjoy photography to start. I always took 200 to 300 photos over a 2 day con, I was doing that with my crappy Lumix point and shoot ten years ago, and I still enjoy it. But obviously you know if you enjoy it or not, but it helps to know you'd do it even if no one was offering to pay you.
-Have a camera you enjoy using, and practice a lot with. Something you don't need to necessarily switch out (as you would a phone). I cart around a DSLR (a 6DM2, in other words nothing fancy), and a couple of good event lenses. You really don't need a full-frame DSLR, but you'd be surprised how many people see someone carting around an anachronistically-chunky looking camera with a 24-70mm lens with a big plastic hood and will actually ask you if you'd take their picture. It's something I'd use even if I skipped going to cons in a given year.
-Have a website--I have an irrational hatred of Instagram and social media more broadly, so I just went and made a cheapo website on a free provider. It could be better, but fuck, it looks better than an Instagram page.
-Have business cards for said website. We're probably getting into the unnecessary here, but I started handing out business cards. Not sure how useful it is (I get about a dozen business cards from cosplayers which I almost never look at), but few photographers ever seem to have business cards (I guess they'll just give you their Instagram if they want).
-If someone asks you, have a rate in mind. Wedding photographers charge, without exaggeration, $500 an hour. That seems insane, but fuck, that's the market I suppose. If you quote a fifth or a tenth of that, and you explain that it's what you think your time is worth, I've yet to have anyone who asked to pay me to photograph them decline (then again, most people don't ask). Be upfront about how you'd like to be paid.