Ring size
The best way to find out your ring size is to let your finger be measured with sizing rings (large keychain with a set of rings in each size). A ring should be relatively easy to put on. When taking a ring off a slight pressure should be developed during removal over the lower knuckle.
Your fingers change in size very slightly during the day and the month, so it is good to have multiple measurements. If you are choosing a wide band ring, consider a larger size.
If for some reason you cannot use the sizing rings, measure the inside diameter of one of your rings (with accurancy of half mm). Then measure the circumference of your finger using a string. Having both pieces of information, your size can be easily determined.