We would highly recommend that your ring size (if you don't already know it) be measured by a high street store jeweller. This is the most accurate method. Please try and ensure that when you take your measurement, your hands are at a normal temperature (not too hot/cold which could result in swelling or shrinkage and therefore be an untrue measurement).
The next best thing would be to order a ring sizer (here). Once it arrives follow the above advice, measure your finger, and you have the sizer readily available for another time.
Other methods include downloading the Jason Withers Ring Sizing app on your mobile phone; this allows you to place a ring that already fits you well, on to your phone screen and finding the measurement that way.
Alternatively, use a soft tape to measure around the finger. Don't pull the tape too tight as you will need some room for movement over your knuckle.
If you don't have a soft tape measure, you can always use a strip of paper. Wrap the paper around your finger and mark with a pencil or pen where the paper overlaps. Again, be sure not to make it too tight as you'll need room for movement over your knuckle. Then, unfurl the paper and measure up to the mark you made with a ruler.