colloquial contraction of 'してはいけない', meaning 'don't' or 'you must not', used in casual speech.
そんなこと、しちゃダメだよ。
You mustn't do something like that.
ここでゴミを捨てちゃダメです。
You can't throw away trash here.
宿題を忘れちゃダメだよ、明日先生がチェックするから。
Don't forget your homework— the teacher will check it tomorrow.