Water supply leak?

Cut a short piece off the end of your plastic pipe (assuming you have some slack) and bring it down to a plumbing supplier. Ask them for an appropriate fitting to go from whatever size copper you are using (1/2", 3/4" or whatever) to the sample you have brought.
When fitting, make sure the ends of both pipes are cut square and that both pipes are not out of round - e.g. no big dent near the end of the copper. Use PTFE tape on the threads if its a compression fitting they give you in suppliers.

JD
May 2009