I’m also new to this, but maybe start by getting as much information as you can about the case... Search for news articles (a local library may have copies of print versions if it’s an older case) and court records... read through those... see if there is any footage of the trial and whether that could be requested, maybe through a public records request. You can also try to make a public records request with the police office that investigated the case for the case files and information associated with that case. If you find and read appellate court decisions, you could try to find out who the person who was convicted of the murder’s appellate attorney was. Or you could see if the jail the convicted individual is at has an electronic communication system, and you could email the person directly and tell him what information you know or have heard, and offer to talk to his attorney. If you find his attorney’s information, you could also reach out to the attorney’s office directly and explain that you have information that you think could help free a client he represented (and give the name). If his previous attorney isn’t interested in helping the person further, if you communicate with the defendant directly, you could offer to help reach out to like the state’s public defender office, or a law school clinic, or a innocence project group to see if they would help with his/her case. A lot may depend on how old the case is, and whether the convicted defendant has exhausted his (regular) appeals. But I hope that gives you some ideas. I’d love to hear more about what case you are referring to specifically.