Ex Girlfriend trying to give up our son for adoption without my consent. How do I get custody?

My ex refuses to discuss the idea of me raising our son and is still attempting to give up my son for open adoption even after I told her I am willing and able to take care of him and that I refuse to give my consent. I got ahold of her grandmother who told me my son is "with a local family who has a plan for an open adoption" and that if they can't complete the adoption they'll just take him home with them. I was not even told of him until weeks after he was born. I had heard a rumor she was pregnant and when I asked her she said she was not. She has asked me to stay away from him and does not want to go to court over this. She also later said that I already did know she was pregnant because she did a test in January that was positive (but she also said a few weeks later that she miscarried). She has been telling people that I knew about the pregnancy and wanted nothing to do with the child, which is a lie. I want to take care of and raise my son. What should I do?