might be easier to do it the other way round
scp remoteusername@remotecomputerIP:/path/to/file /path/to/dir/forfile/
and honestly, if you don't know your own username, you are in trouble.
I accept that finding out your IP address might be a challenge
a websearch for "whats my IP" might help
configuring your router to forward ports, refer to router manual
port forwards are best left alone unless you know what you are doing