Modern History is generally considered to be any thing/event post Enlightenment, some consider it to be as far back as the late Middle Ages. You can't simply get a decent overview or the basics since it covers so much. E.g. British modern history will be different to Egyptian modern history.
But I would say knowledge about the major wars is a decent start, but even that might have a country-specific focus.
You could also look at the HSC Modern History syllabus and look at the topics they cover and have a read of those.