To find out your correct ring size, you can use the Ring Sizer (Jason Withers) app for iOS / Android. While there may be slight inaccuracies, it should still be very helpful. If you already have a ring that fits well, place it on your phone screen and make sure there is a small gap between the ring and the blue circle—imagine that the blue circle is your finger, and it needs a little bit of extra space, as shown in the picture on the left.
All ring sizes in the shop are based on the inner diameter of the ring. You can also measure the inside diameter of a ring you already own using a simple ruler.