In many airports, you go to the plane down a long tubular corridor, which butts up against the side of the plane. Great if it's a warm, sunny day. The first thing that might surprise you about getting on a plane, is that it is surprisingly large, and surprisingly low-tech. Also, a lot of airlines' planes seem to look pretty shabby. I have faith in the international inspection system.
Larger planes have two aisles, and small ones have one.
When you get to your seat, stow your hand luggage in the overhead locker. You can keep it under the seat in front of you, but it's a real nuisance not to be able to stretch your feet. Apparently, people swell when they fly. I have no scientific proof of this, but a number of places have reported that it's the case. Consequently, loose clothing is order of the day.