Consider the following sentence.
What does the 「もんね」 mean?
Without もんね、i.e. 「お金持ちになる。」, it means "becomes rich"。
「～～もんね」 is a sentence-ender that is used mostly, if not exclusively, by children. Technically, both 「もの/もん」 and 「ね」 are sentence-ending particles, so the two are combined in 「もんね」.
It is used to state or declare something boastfully, braggingly, etc. often with a hint of playfulness.
"I'm gonna be rich; That's for sure!"
"I'm gonna be rich, I promise!"
As an adult, you could use 「もんね」 kiddingly once in a while, but if you used it in a serious tone or used it too often, you would end up sounding pretty weird.