arrive early to get through security since you will be unfamiliar with the process, review the procedure online so you are prepared
Enjoy the airport atmosphere after security, the shops, restaurants etc relax and people watch, have a drink but limit alcohol (a relaxing glass or two will benefit you but no more) and caffeine to 1 cup, otherwise it will make nerves worse.
A good app is flightradar24 to see your flight path and see how your route travels in the days before your flight, great for us geeks.
Check seatguru for your aircraft features like toilets, entertainment and good/bad seats.
Your carry on should be packed from bottom to top with the following: a warm item of clothing eg hoodie and warm socks, Charger, small snacks in case inflight food isn't to your liking, alternative entertainment like a kindle or iPad, your liquids bag with lip balm, eye drops, face wipes, moisturiser on top for easy access. Keep passport and boarding pass in a small pocket if possible to avoid the dreaded search under pressure �� and they have migrated to the bottom of the bag.
Whatever you pack you have to carry.
Once onboard relax in your seat and if boarded early enough get to know the controls and features like recline, tray table, window sash. If possible stow your carry on under the seat in front to avoid disturbing other passengers and for easy access. If not possible put your entertainment in the seat pocket just in case.
When taking off there will be an initial burst of speed which will feel like you are being pushed into your seat, this is scary for first timers but is exhilarating for frequent flyers, expect this and marvel at the feat of engineering of flight ���� don't hold your breath as your ears won't pop, if you suffer from any ear ailment then suck a hard sweet or mint and you will be fine. The plane will climb fast before levelling off, savour the experience as it will never feel the same again.
As a general rule how something affects you on the ground will double/triple in the air, this includes alcohol and prescription mess like Xanax so be careful and start low.
Flying is a marvel and enjoy your first flight.