When is it appropriate to use the polite forms "Ms.", "Mr.", "Mrs" and how exactly should I formulate them? I seem to be confused in the American English and British English traditions in that regard.

As for the usage in America/Canada, it is generally accepted that Mr. is for any male you want to refer to politely. For women, Mrs. is only acceptable if you know they are married, Ms. if you are unsure, and Miss if they are unmarried. When I was a child I was taught to speak this way to my friend's parents, or to my teachers (for example, Mr. and Mrs. Johnson). As an adult, I rarely ever use these titles, as it's often considered too formal for two adults (considered equals, in most cases). It's common for two strangers to introduce each other by first names only, or to address someone by their full name (first and last name), and from then on go by first names. It really depends on the person, but I would say if you are younger, you can safely call someone much older by the Mr/Mrs/Ms title if you don't know them.