How to Find Your Ring Size
Determining your ring size can be tricky and my recommendation is always to go into a jeweller to be sized professionally.
Each finger on each hand is a different size, so make sure you measure the finger you will be wearing it on (or all of them, for future purchases!) Also - wider bands fit different - so make sure you try on a few of those also.
The ring should fit your finger comfortably, being snug enough that it wont fall off - but loose enough to slide over your knuckles without discomfort. Your finger size can change depending on the time of the day and the weather - heat making your fingers swell and the cold making them shrink. If you are hesitating between two sizes - always choose the larger one.
Using a ruler and a piece of string, or an older ring which fits the finger you want your new ring to fit, or using a measuring tape are a few more ways to determine your ring size.
There are also a number of downloadable tools on the internet to print and find your ring size
start here - http://www.ringsizes.co/resources.html