how not to be toxic in any game

flame irl, don't type it.

play with personal improvement in mind and don't mind what others are (or aren't) doing.

Internalize the concept that this is just a game, and that you're gonna lose an average of 50% of the games you play.

Somebody performing poorly on your team isn't a reason to allow yourself to get overly upset and flame them, and if somebody is trying to troll you, simply mute and report them. The worst they can ever do to you is ruin one game, which you already had a 50% of losing fairly anyway. Shit will happen.

I got my first ever penalty in 7 years playing this game

Uncheck "All Chat" from the game. Use Mute /all if you are getting triggered. If you are ready to type in chat, spam ctrl 4 (champion laugh.) Problem solved. I let my champion laugh at me for trying to be toxic in chat if any. It surprisingly works.