Forget about this website, forget about what an incel is, forget about this whole damn thing. We'll be here for you if you fail, but if you really do have a chance being immersed in incel shit here might make your success less likely (Blackpill = thinking you'll fail).
Get drunk, it lowers your inhib. Be enthusiastic. Be "cool", bring some beer or weed or whatever they're into. Have one or two canned "cool things" (travels/vacations, bands nobody else has heard of, interesting hobbies) to pull out if you can, if you can't come up with anything it's okay.
The fact that you got into a frat and you're going to a big party is a sign strongly in your favor that you might be a mentalcel. I hope I don't ever have to see you here again, that you'll ascend and forget about being an incel. If you don't ascend, let us know how it went.
Good luck OP