Your ring should fit your finger comfortably, it should be snug enough so that it will not fall off, but loose enough to slide over your knuckle.
Your finger size can change throughout the day. It is a good idea to measure at the end of the day when your fingers are at their largest. Do not measure when your hands are cold, your fingers will be at least half a size smaller.
You can easily use an old ring to measure the diameter of your ring inner circle. Alternatively you can measure the circumference of the thickest part of your finger that you want to use for the ring. You can also find the conversion from diameter or circumference to international codes in the below table.
You can also find below the international ring size chart.