If you don’t know your exact size yet, you can use a simple method:
Wrap a string around the second phalanx, at the widest point of your finger. The string should not be too tight or too loose. Mark with a line where the string meets. Finally, unwind your string and measure with a millimeter ruler the length necessary to go around your finger. This distance in mm corresponds to your finger circumference. You can also use a strip of paper one cm thick.