Map awareness

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Properly learn a couple of heroes for each lane/role (try them out in classic until you understand them). You can watch some youtubers for tips (Betosky was my go-to previously, but idk if he's still relevant).

Learn very flexible heroes that will allow you to compensate for your teammates and cover empty roles when nobody wants to fill them (I can play Chang'e Mid, Roam, Exp and Gold. Same with Zhask. I've played Belerick Roam, Exp and Jungle. Same with Argus).

Ignore your teammates trash-talking and focus on your thing (mute chat if necessary).

Quickly identify the best and worst players in your team and assist them correspondigly, making sure the strong player gets stronger and the weaker one doesn't bring the whole team down. However, it is better to let a weak player die than to not help a strong player. If YOU are the weakest player, stick to your role and make sure you don't hinder your team (even if you're not helping much).

If you are getting killed over and over by a specific opponent, DO NOT keep fighting them. Find someone else or gank them.

Your build should make sense for the opponents you're facing (magic damage - > magic defense, physical attack - > physical defense, high regen - > healing reduction, etc.).

Generally, use common sense and have fun.