The size of a ring is measured by by its inside circumference.
Thus, a size 56 ring is a ring whose inner circumference measures 56 mm.
This measure should match the circumference of your finger.
To take this step, use a paper tape or string, go around your finger, and see the measure on a ruler. So you get the size of your finger.
This method is approximate. If you have the opportunity, go to a jeweler to know your exact finger round.