It's important to note that:
* "Monsiour" is not a correct spelling. The correct spelling is "Monsieur."
* The pronunciation is similar to "moh-syuh."
* It is always capitalized.
You can use "Monsieur" in a similar way you would use "Mr." in English. For example:
* "Bonjour, Monsieur Dubois." (Good morning, Mr. Dubois)
* "Merci, Monsieur." (Thank you, sir)