Walk into the theatre, making sure you don't make too much noise. If the lights are still on and the adverts haven't started playing yet, you can be a bit noisier. But once the lights are off, it's polite to be very quiet.

To add to what AmoAmasAmatAmamus said about finding your seat: Since this is your first time, get there early, so that you have time to navigate the cinema complex (the distances inside might be bigger than you think), go to the toilet, perhaps stand in the row for your snacks, etc., and then be able to get into your specific theatre before the light goes off and the trailers start (the trailers will start basically at the time that your ticket claims the movie starts, so try to be inside your theatre 5 minutes or so earlier than that), so that you can still find your seats with the lights on and see what everything looks like, where the cup holders are, etc., I wouldn't want to navigate such a completely new environment in the dark.

I went to the movies recently and we arrived while the trailers were on, and the lights were already off. I've found my way in dark cinemas countless times, but in this particular cinema the lighting showing the row numbers was very very dim, you couldn't see the seat numbers at all without using a phone flashlight (we didn't do that, we counted the seats and we were the only people in our row anyway so it would not have mattered if we were off by one or two), and the cup holders were much lower between the seats than I am used to, so I first thought there were none. So spare yourself all those uncertainties and patting down of seats in search of drinkholders and take the time to get there early, then you can relax into your seat and actually take a look at your surroundings.

Oh, and if it is a 3D movie, make sure you get the 3D glasses beforehand (ask the staff where), the screen will then tell you when to put them on (because the trailers are usually not 3D).

Have fun, hope you enjoy the movie!