First, be sure to bring plenty of cash - you'll need at least $8.00 (US) unless you're attending a "matinee" showing (which means the movie starts before 4pm). Matinees are usually like half price.
If you're planning on having any refreshments during the show - drinks, popcorn, candy, whatever - you'll need to bring cash for that, too. They frown on you bringing your own stuff. Everything at the movie theatre is way more expensive than it is anywhere else. So you need like $10.00 for a coke and popcorn.
So, when you get there, go up and stand in line at the ticket booth to buy your ticket. They'll want to know which movie you want to see. You tell them which movie and how many tickets you want - adult, children, or senior citizen (different price for each). they'll tell you how much to pay, you give them the money, then they'll give you a ticket.
Then you go stand in line at the snack counter to get your refreshments. A large drink is like a gallon - a large popcorn is like a 5 gallon bucket. No one can possibly finish all that in 2 hrs, so it all gets dumped out anyway. So don't be afraid to order a smaller size even though you feel pressured to get the big size - it's just crazy. They'll also encourage you to let them pour like a gallon of melted processed butter-like liquid all over your popcorn - i tend to skip that, because it's really messy.
This whole process may take up to 30 minutes, depending on how long the lines are. So be sure you get to the theater early enough to get through the lines.
Once you're all set (have you gone to the bathroom? - do that now) head toward the theatre where your movie is showing. There's usually like a long hallway, and each theater has a big sign over it that says which film is showing in that particular room.
Go inside and find a seat - again, this is why you want to get there early. You want to get inside and find a seat while the lights are still on. Once the movie starts, they turn the lights off, and you can't see very well.
Don't sit too close to the screen - you'll spend the whole time looking straight up, and your neck will hurt. Pick whatever seat looks comfortable to you. Some people like to sit next to the aisle, some people like to sit right in the middle. The screen is designed to be seen from all angles, so it doesn't really matter. Hopefully some tall guy won't sit right in front of you and block your view. Most people will find it awkward if you sit right next to them if there is room to sit further down - leave a seat between you and someone you don't know. It's ok to sit right next to someone you came with - even if you are both men.
When the movie starts, be quiet. No one likes to hear people talking in the movie. Some idiots seem to think that their voice can only be heard by the person right next to them that they're talking to, but of course it can be heard by everyone in the theatre. I guess they're too used to wathcing movies in their living room and have forgotten their manners. It's ok to tell them politely to shut the hell up, or even throw stuff at them if they don't - that's where the popcorn comes in handy.
Don't put your feet up on the seat in front of you. That's considered rude. If you're there with a date and you want to grope each other, it's probably best if you sit way in the back so that no one has to watch you and get distracted. If you're planning on engaging in any sexual activity, be sure to bring some tissues or wet-wipes or something.
When the movie is over, it is ok to applaud. Although no one who actually worked on the film can hear you, so what's the point? People do it anyway, so feel free to express yourself. It's also acceptable to cry, even if you are a man. Another good reason to bring some tissues. That and the popcorn butter.
As you exit the theater, be sure to take all your trash with you - empty cups, candy wrappers, popcorn buckets, used tissues, etc. A lot of people just leave the stuff there for someone else to clean up. i think that's kind of rude, too. there are plenty of waste bins right by the door to throw the stuff in.
You may have to use the bathroom again - that gallon of diet pepsi needs somewhere to go. Be prepared to stand in line again - everyone else has to dispose of their pepsi as well. Be sure to wash your hands when you're done.
That's about it. Have fun, and try not to think too much about it!