Is there a simple way to turn formal words into their informal, natural way of saying them?
I am learning french and recently met two french guys my age at a summer camp in Los Angeles. I can speak simple things in french yet they always noted on how formal and unnatural my way of talking was.

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There is things we don't usually say whan speaking that could help: we don't say the "ne" in negative sentences
"Je ne sais pas" => "Je sais pas"

Another thing might be the vocabulary you use, if you learnt it at school, you might have more formal vocabulary than what people actually use...

Answer

Yes, textbook french is very formal.

You can look at french peoples' social media and see "argot"
or you can find a french book on argot. You need "argot" to get informal

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