One of the easiest ways to assess your ring size is to measure your finger with a strip of paper or string. Start by cutting out a strip of paper that is approximately 3/4-inch wide and 4 inches long. Wrap the paper around the base of the intended finger, and then mark where the paper overlaps. The length of the paper you measured in millimeters is the circumference of your finger. After finding this measurement, use a conversion chart to find the corresponding ring size. If the size of your finger below the knuckle drastically differs in size from the knuckle itself, measure your knuckle as well and choose a size that falls more in the middle of these two measurements.
If you have a ring that already fits your finger well, you can use a ruler to measure the inside diameter of this ring. Just take your ruler and measure straight across the inside of the ring from edge to edge. Take the diameter measurement in millimeters, and then a measurement-conversion chart to find your ring size.
I also offer to ship out a small ring sizer tool for people who are planning to purchase a ring from me. Just send me an email to firstname.lastname@example.org. I will cover the ring sizer and you the postage!
The best way to get properly sized is however to visit a jeweler or goldsmith that can help you out.