Starting an OCS Packet?

I am currently enlisted in the US Army. I want to start the process to apply for OCS. I have been talking to my chain of command and getting a few different answers. To start my packet, who do in need to talk to? I have been told that I need to go to my unit's retention office, but I have also been told I need to go to Post Retention Office. Can anyone please clarify where I need to go to start the process?