Being a former pilot for 32 years with United, I can maybe add a little something here. First, when the Flight Attendants (F/A's) close the doors, the pilot will pressurize the cabin. This saves the passengers from a number of environmental disorders and even your brain playing games with you. Without pressurization, the passengers and crew would probably experience Hypoxia and other disorders like decompression sickness almost the same as is experienced by deep sea divers when surfacing too quickly. Everyone here is right, you should drink plenty of water. This will prevent jet lag. The air in the planes are dry, but clean, despite what you have heard. The air is continuously filtered and has been known to be as high as 95% free of pollutants that can cause disease. I would steer clear of alcohol. When you are able, get up out of your seat, walk up and down the aisle to prevent clots from forming. You are going on a long flight and inactivity is a bad thing. At the very least, wiggle your toes and stretch while seated. Flying south to north and vice versa, you may cross the jet stream and when going through the clouds, there will probably be some turbulence, which is no big deal, just uncomfortable to some passengers. The plane will normally make some strange sounds to you. This is nothing to get excited about. The landing gear coming up and then going down is the one that scares most first-time flyers. Flaps being extended and retracted also makes strange sounds to new flyers. If you see the F/A's up and walking around the cabin during the flight, all is well.
Like most others have already said, flying is safer than being on the road. If you are really excited about this first time trip, you may want to see your doctor and get a tranquilizer, but do not drink alcohol with it and as I already said, don't drink alcohol anyway. Sit back relax and enjoy the trip and the experience. Flying is a beautiful thing. If headphones or earbuds are available, put them on and listen to some nice music, or watch the in flight news and movie, if available on your flight, or you may wish to take a pair along. You may even get lucky and sit next to someone that likes to share in some light conversation. And last but not least, if anything is making you uncomfortable or you are feeling anxious, tell the F/A. They have been trained to help you.
Happy Landing.