Have you been eating asparagus, or spinach or another food you wouldn't normally eat? Or have you started a new course of medication/vitamins? These are the usual causes af green urine - if the answer is no to both questions, or if you have other symptoms, I would recommend a trip to the docs to get a sample checked out.
As previous answers denote, it may depend on what you've been eating or drinking. It may simply be clorourants in the food or drink. How do you feel otherwise? Do you feel tired or weak or do you feel all right? If the former, see your GP.
Assuming you have not eaten anything recently which would have the effect of a dye (asparagus/spinach/beetroot/ribena etc) the most common reason for green urine is dehydration. normal healthy wee is straw coloured, but becomes greener/yellowy when the body starts to dehydrate as the kidneys work harder. drink plenty of fluids (a couple of glasses of water at least) and you should notice your wee returning to a nice healthy colour within a few hours. if it doesn't, see the doctor