I totally understand how stressful it is travelling for the first time with your child. Whether they are 18 months or 5.
Our first trip with my son (he was 11 months) was stressful, I didn't know if he would sleep on the plane, cry, if his ears would hurt him etc. I packed for every single scenario. And them he surprised us and was fine. My point is there's no way to predict how your child will react but I totally understand the stress.
A few pieces of advice:
First, before you board the plane let your kid play, walk around, etc. Don't make them sit still. They're going to have to once they board the plane. Most airports have a play area, we would always find it and let our son race around etc. so once we boarded it was like going on a car ride, you have to sit still-ish etc.
Second, if your child is still in diapers take 1 or 2 and a small wipes case and put them in a ziplock, etc and then put it in the seat pocket in front of you. This way if you do end up having your change your child on board you don't have to go through your entire bag to find your essentials.
Third, if you are travelling with both a car seat and a stroller make sure you have a bag for both. It makes carrying them through the airport easier. We had a duffel type bag for our Britax that worked great (it was a specific car seat bag) so we could carry it over our shoulder. When we boarded the plane we would take it out of the bag. Also, we would put our stroller into it's bag prior to having our tickets scanned and going down the jetway. There are always a lot of families packed there trying to collapse their strollers etc before they board and by having your stroller already collapsed and in a bag it saves you a few minutes. Plus like suggestion #1 it gives your child the opportunity to walk down the jet way.
Best of luck to those of you flying for the first time, I'm sure it will be great and you'll be pleasantly surprised