Polite
having or showing behaviour that is respectful and considerate of other people.
https://www.lexico.com/en/definition/polite
Polite means showing regards for others in manners, speech, and behavior. ... The adjective polite comes from the mid-13th century Latin politus, which means "refined" or "elegant." Showing consideration for others, using tact, and observing social norms are the qualities of being polite.
https://www.vocabulary.com/dictionary/polite

Let's see kind of person you are, by looking at your games: If you oftenly play one of these games, I don't say you should stop, but please, stay away from TF2 -
Counter-Strike:Global Offensive;
Call Of Duty;
Dota 2/League of Legends;
Fortnite;
in rare cases Overwatch.
You may cause bad influence on people you play with by your "salty" behaviour. If you don't play any of these, or you can keep yourself cool and good, continue to read this guide and play TF2.

Raging at people in voice/chat

1. Let's say your team has a Medic in pub(wow), and he only heals his bf from your perspective. What do you do? Instead of trashtalking on him for "not healing you", try to calmly ask him for paying more attention to other members of the team. Personally, I wouldn't heal someone who doesn't understand that I'm the only healer this team has, and someone isn't my "top priority" for healing. You insult me for trying to do my job well, so why Medic on my place would heal you? Think about it next time you want to talk bad about your helpful support dude.
If the dude is new, try helping him. It's always nice to see how new guys learn everything. He may remain silent, but much appreciate your advice.
2. Other team has this very good Sniper, and you can't stand your anger, almost writing another trashtalking message in chat.., Instead, try calmly articulate the fact of your frustration into a compliment. Sniper may remain humble and silent, but he also might take a note from your behaviour and be a better influence on people. More kind things you say to others, better they be to others.
3. Pretty much the same thing as in 2. can be articulated to all classes, try be polite and they'll be too. Hopefully.

Going hard on someone

1. Let's say you got this "ultra tryhard Soldier" on the other team, and he seems to mainly focus you. Of course, you can try to pick a counter to him, for example, Pyro(if you're good enough at pressing M2) or bbox+c Soldier. But it's not honourable at all and boring. Try baiting him on Market Gardener fight! It's fun for both of you, it's hindering you much, and having a duel, even if it's silly, is polite. Of course, it may not work, but you both can have fun trying to do something like that.
2. As Sniper, killing other team's Sniper is primary objective, but some people go way too far with it. If you're "terrorizing" enemy Sniper, try giving him an advice. This may not work first time, but next day he'll be calm again, remember your tip, try to use it, and it might work for him. He'll be thankful.
3. This can be said about pretty much every class, so take it as general polite tip
4. Don't be too harsh with new players! If you ever get into situation where you're the only aware, help them by giving tips. It sounds like stupid idea(and it is actually), but maybe one of them is going to take your tip. Leaving match in this case is the worst you can do, so try helping out new, poor souls.

(oh god I'm running out of ideas)Taunting, using certain weapons, etc

1. You can't surely know if someone has a bad day and super upset, but instead of making his situation x100 worse, try cheering him up. Give him a couple of kills on you, let him loose one of his dominations, even if he didn't deserve it. You can get more later, but he? - Probably not.
2. I personally don't appreciate how many people wear "super edgy cosmetics" and rename their rifles/hats something insulting or making them look cooler, it's all just window dressing. It's only my concern, so you can't skip it, but please, try making your presistance less "cool", you only make a fool out of yourself. Otherwise, if a guys is "edgelord" and upset, don't be harsh to him.
2.1. Actually, be harsh, I ♥♥♥♥♥♥♥ hate those guys more than anything, so I don't care about their 12yo dreams getting destroyed by someone owning them. ♥♥♥♥ you if you're doing this and think you cost something, there is a special treatment in hell for your type.
3. Someone may not like your choise of weapons, maybe because it's too easy to use, or he just hates to fight against it. Thing is, switch to something that isn't ♥♥♥♥♥♥♥ Phlog or bbox+c. switch to something else, if you're Pyro, try out use shotgun more, or Direct Hit if you're Soldier, Huntsman, Baby Faces Blaster, Loose Cannon, whatever, just make your gameplay different.
4. Insulting nickname is the same thing as 2. 2.1. , so try to avoid that. Or one day, you might be the guy that get's hardly punished.
5. (I've completely ran out of ideas at this point, so) Don't be a ♥♥♥♥ in general, taking someone's healthpacks when they're about to die, or not extinguishing teammates when they're low-hp, that kind of stuff. It's not nice, therefore, not polite at all.

Personally, I have no idea what the ♥♥♥♥ I just wrote, but try it out, and maybe we'll make community great again. Who knows. I'd also appreciate if you join my stupid steam group about birds and stuff, there: Sir Hootsalot Gang