What should I do and avoid when visiting Europe?
The answer already posted is excellent advice. I would add a few phrases: Where is the restroom; Where is the train station; I'm sorry, I don't speak (your language); Do you speak English (or some other that you are fluent in).
The inquiry about the train station applies even if you're not traveling by train. In many countries in Europe the train station has a kiosk or counter with info about sites to see, rooms not rent and lots of other really useful stuff. Other than the week we stayed in Paris, we did 2 weeks in rural France without reservations, simply booking locally from notices in the stations - this was in the shoulder season, however.
Also, if you know your itinerary, do as much internet exploration as possible and print out specifics of museum hours, recommended restaurants, etc. If you are going in the summer (or pretty much anytime in Florence), buy tickets to museums and major tourist attractions in advance. Some bookings can only be made in advance - like the Borghese Gallery in Rome. Check the currency exchange rate a few days before you go Notify your bank of your dates of travel and ask them what European partners you will be able to use to access ATM withdrawals. If possible, get a credit card that won't jack up your fees for currency conversion (JCB has been a reliable Asian card in Europe for some time - I'm unfamiliar with others). If traveling with a chip card, get an RFID-blocking wallet or purse. Buy travel clothes with zippered pockets and keep what few valuables you carry on the street in them. Skip Airbnb for a small local hotel with a 24 hour front desk and some sort of room safe to secure your valuables. Subway stations are particularly likely for teams to work the tourists over. Walk away from a subway car that has a group of people actively blocking you in some way, as they may be maneuvering you into a crowded situation to pick your pockets clean with a lot of jostling.
You should make an effort to learn some basic phrases in the languages of the countries you will be visiting, this way you can introduce yourself politely, explain that you don't speak the local language and whether its possible to speak in English or any other language that you may know.
I have had the pleasure of visit many countries on 5 continents, I have lived in multiple cities in four continents, and I can assure you that good manners go a long way in making you a welcomed guest. Make use of what the french call "politesse": excuse me; please; thank you; your welcome; etc. (in the local language) accompanied by an open smile and you will discover how basic consideration will open doors for you around the world.
Beyond basic language/phrases, one should also familiarize oneself with the customs and idiosyncrasies of your destination before departing on your trip, in the internet age ignorance of local customs is the result of laziness. Remember that you are the guest in their country, therefore it behooves you to show respect and adapt to local customs.
Be polite and learn some of the basic language. Stuff like hi, how much is that, my name is, where is the bathroom, etc. Just like if a American visiting Korea. If a Italian came to American and tried to speak English I would try harder to help them. Dont take a picture of everything. Buy a postcard, send it home. And yes Europe drives on the left side of the vehicle/ right side of the road. You can rent a car and drive around the block. Get use to pulling into traffic from a parking lot. Just remember the center of the road is on your side, driver's side. Just like driving on the left side.
I hope this helps