I've traveled internationally many times over the years, and there are a few things I've found to be extremely helpful — and a few things you should avoid.
Let's start with the don'ts
Don't be rude to airport and airline staff. It's frustrating when something goes wrong with your travel, but they're not the people to take it out on — they want to help you as much as you want to be helped. It's not their fault. A little kindness goes a long way, and who knows, it might even help you get on that sold-out flight or land an upgrade. It's never a guarantee, but you never know.
Don't act entitled. When you're traveling internationally, you're the guest — no one owes you anything. This goes for hotel staff, taxi and rideshare drivers, and any locals you meet. You can't expect everyone to speak your language, so be patient. Speaking louder doesn't help someone understand you better.
Now, the do's
Be kind. As mentioned above — it goes a long way.
Be aware of your surroundings. Use your gut. If a place looks unsafe, there's a good chance it is.
Dress respectfully. Some cultures have different norms around dress. That doesn't mean dressing to the nines everywhere you go, but it does mean being aware of where you're headed — sometimes it's as simple as packing an extra shirt or a cover-up. And to be clear, this isn't just about women — plenty of countries have dress expectations for men too, like a head covering or long pants at certain historical or religious sites. A little research goes a long way.
Check your phone plan. You don't want to be caught somewhere with no way to contact anyone. Do a little research ahead of time — a local SIM card is often far cheaper than using your plan from home.
Remember your passport. This is your ID while traveling abroad — you'll need it to enter the country, and some places even require it to rent a car or drive. If you're out exploring, it's good to have something to identify you. That said, you don't need one of those money/passport pouches that scream "I'm a tourist" — a simple zippered bag will do.
Bonus: carry some cash. This one's more of a suggestion than a must, but having cash on hand is useful for entry fees, tips, or last-minute essentials. Depending on where you're headed, some places only accept newer bills — 2017 or later.
Bonus: get familiar with WhatsApp. In a lot of countries, WhatsApp is the go-to for communication instead of texting or calling — worth downloading and learning before you go.
This isn't meant to be all-encompassing — I could probably think of a dozen more things I've picked up along the way — but hopefully it's a good starting point. When in doubt, ask.