You are right, almost everyone has such a IP address, it is a 'local' IP address: within a local network, home or business, not the world wide web. There you have been assigned an IP address by your internet-provider. You can view this address by visiting http://whatismyipaddress.com from your computer.
That will give you the IP address you are looking for. Then you will most likely have to tell all network routers in between the internet-modem en your PC that they should direct all traffic to your PC; this is called port-forwarding, I believe there is a section on that in the fileMaker documentation, otherwise a search on their site will give you more info. And you will probably have to visit your firewall settings as well. Again, the FileMaker IWP documentation has something on this, I believe.