Just to build on kindeye's advise.
Talk to them. Stay Calm. Don't shame or blame. Establish boundaries. And allow them to feel the full force of the consequences of their use (legal, school, etc.) without protecting them.
I would add - avoid anger. If you lose your temper - you have lost the conversation. Its really a disease (a disease of choice a kind of brain disease). I don't think punishing the addict is the answer not is enabling them. Let them experience their own negative consequences.
The whole idea is for them to seek treatment and recovery on their own - whatever that may be AA, NA, out-patient, in-patient, rehab etc.