Movies are terrible first dates. They’re terrible in general for dating. But if you must go to a movie with someone you’re not already in a long term relationship with, then here’s the skinny.
You have two things to pay attention to. Your date and the movie. If you pay too much attention to the movie, then you won’t have gotten to know your date very well. But if you don’t pay attention to the movie at all, then you risk looking desperate and awkward.
My advice is to get physical from the get-go. Don’t wait until halfway into the movie to start holding hands or to put your arm around her. Do it as soon as you sit down. Watch the movie with her but also pay attention to her and get to know her on the non-verbal, just holding each other level.
Kissing is a good thing to try after an action scene or right after the climax of the movie. Just reach over and with one finger, turn her face towards you, then lean in. If she resists you don’t push it. Just try again after the movie is over. If she resists you twice then she’s probably not into you and unless she’s giving you a lot of other signs that she is, don’t pursue it.
As for the question you actually asked, lol. You always want to look your best with someone you’re just getting to know. Pants and a dress shirt. Later on after you have been dating awhile you can dress more comfortably. You may not be comfortable in the clothes, but good style can erase a lot of other faults. The main thing you want is for your clothes to fit properly. Excess fabric or not enough doesn’t look good on anyone.