If you know your ring size but need to convert it to UK sizing, please use the table here.
If you don't know your ring size at all, you can either purchase a ring sizer or measure your finger and try to calculate your ring size.
To measure your finger, take a piece of paper and wrap it around your finger. Place it against a ruler and then use the table to convert it to find your ring size. Please note that this method is not as reliable as using a ring sizer.