Secure WCF service and Methods

Question

I have created a WCF service in our Project. There are many methods inside this. Now its opened. That means anyone form outside can create a proxy of our service and could easily use our methods. How to secure this thing. I used BasicHttpbinding and read on the Certificate security. But not able to make secure. Please guide me. Its urgent. I cannot change the basicHttpBindings. So give me a solution to protect my service