This sounds like your heater core might be leaking. I have included a few links for you below. Please go through these instructional troubleshooting guides and they should help you track down and fix your problem. Let us know what you find out.
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.