First ever meeting invite - any advice?

[phosphoenolpirate]

Probably about a million of these posts out there already but on the off chance somebody sees this...

I've been sending out emails for the past couple months to no avail, and just when I was beginning to lose hope, a prof emailed me back and asked to meet in person to tour their lab and get a feel for the program, discuss my interests, etc.  I'm super excited but super nervous.  I graduated right when the pandemic started and never did any undergrad research (I got way too absorbed in my ECs and academics, my mistake obviously) so I hope that doesn't make me look bad? An MSc is something I've always debated doing but it was one of those half-formed plans while I focused on my more thought-out ones (which, obviously, got derailed, thank you Canadian medical schools for your refusals, very much a villainess maker moment) and now here I am, armed with nothing but a dream, a whole bunch of notes on research ideas, and a half-full cup of iced coffee.  

So, basically, what do I do.  Like.  Do I go in with a vague proposal? Scale of 'business casual' to 'entering the C-suite in a power suit as I announce my takeover', how dressy should I be? What kinds of questions should I be asking? I don't even know what I'm trying to ask here, I just needed to scream into the void somewhere in the hopes that somebody stuck in the void with me will scream back.

tl:dr am new to this and extremely directionally challenged, would appreciate some guidance, WoW (Words of Wisdom), a Humbling....etc, etc., thanking you kindly in advance.