How can I automate port forwarding?
I'm pretty sure I understand what you are trying to accomplish, but saying "I want to forward a computer to port 22" does not make any sense. You are trying to forward incoming packets addressed for port 22 from your router to your Mac, is what you mean to say (I am assuming). In any event, port forwarding is done in your router's settings, not your Mac's.

You can look up port forwarding steps for your router at http://setuprouter.com/.