do not put anything in the overhead storage unless it's a suitcase
if you're bringing a backpack, put it "upside down" under the seat in front of you. the floor is gross, don't put your backpack down and then put that grossness against your back when you get off the plane
if you're in the back half of the plane, be ready to volunteer to move to an exit row just in case the opportunity presents itself before the plane takes off
if you're scared of flying, know that the plane is shaped physically to stay in the air. Why do paper planes fly? because of their shape. Real planes are shaped waaay better.
only get water, no ice
write down where you parked or take a picture and know how you'll get back
have your first meal planned when you arrive at your destination
To add to #5: safety margins are kind of nuts on airplanes. I want to say a lot of mentally deficient rednecks build airplanes, but that's confirmation bias. Most of them are decent. Even then, safety margins, quality standards and procedures are made to account for morons and cowards that don't want to admit they fucked up. The critical bits are done with even higher standards.
They're safe as fuck. I've been in the industry for almost 20 years now and I love flying. When the plane first banks after take off and the whole airframe creaks is my favorite part, even. Fuck you nature: we fucking win.
The thousands of people across the globe that were involved in building the plane you're getting onto are proud of what they do and genuinely care about your safety. Smile and tell gravity to suck your dick.