Eat whatever you usually eat beforehand (that makes you feel physically good/normal) so you have energy. Don’t be super dehydrated but don’t drink hella drinks either, since it’s kinda harder to get to the bathroom - you could also eat some salty snacks that day to not have to go quite as often. On the other hand, do use the restroom if you feel you need to. You want to feel comfortable!
Research what you can and cant bring into the country/countries you’re visiting. You will need to go through customs (sometimes they hand you a slip to fill out on the plane) and declare what you have. Looking into the process beforehand will help you feel more prepared when you get the official forms.
Get to the airport with plenty of time. Double check that you have passport and anything else you need (use a packing list). Find out whether it is helpful to sleep on the plane in order to be more in line with your destination’s time zone when you get there. Bring earplugs, an eye mask, and maybe an inflatable neck pillow so you can sleep.
I know this one is everywhere, but don’t leave your stuff unattended. And that doesn’t just go for the airport, either. Don’t leave stuff unattended anywhere except your hotel room, and only do that if you have the privacy sign on the door. Also don’t trust anyone who offers to take your photo out of the blue, many of them steal your camera/phone