What does there now mean here?
"There now! What did I tell you?"
And when can I use it like this?
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https://www.merriam-webster.com/dictionary/there (definition 5).
'There' is used in various exclamations with no logical meaning, such as 'There, there' when trying to comfort a crying child. In your example it means 'See - it's happened just as I said it would!'.