Converting units of measure used to be really difficult to learn, since you had to use conversion factors that were different from each other. For example: 12 inches in a foot, 3 feet in a yard, 220 yards in a furlong, 1760 yards in a mile – ugghh!
In the metric system, the units relate to each other in multiples of powers of ten. So there are 10 millimeters in a centimeter, 100 centimeters in a meter, 1000 meters in a kilometer, and so on.
A note about spelling: If you live in the United States, your spelling of these units is based on “meter”. For teachers in the UK, Australia, Canada, New Zealand, and many others, we spell them using the base unit spelled “metre”, based on the original French spelling. Either way, they mean the same thing!
Many people learned to do calculations like this by moving the decimal point. This is a confusing idea, and should be avoided. Rather, talk to students about moving the digits to a new place; you can use a place value slide to show them how easy this is to follow.