A ring should fit comfortably on your finger, so it doesn`t fall off, but can slide over your knuckle.
The best way to determine your size, is to go to a jewelry store and check your size there, or buy a measuring tool, they are available to buy online.
You can also measure an existing ring that fits you well, and check against the attached conversion table.
Please measure the diameter inside the ring.
Or you can measure the circumference of your finger, this tends to be the least accurate, as it`s hard to check if it will slide over your knuckle.