Mel, it's not stupid, just something that's new to you. It was new to all of us at one time. IP addresses of the form 192.168.x.x are private IP addresses. They are reserved for local private networks. If your router is set as a DHCP server, it assigns addresses to the systems that you own using private addresses. This is quite typical for a router. It allows you to have one *public* Internet IP address (or WAN address) shared among several systems on your *private* local network (LAN).
To SSH to your system from out on the Internet, you will need to know the public IP address that is assigned to your router. You can find this in the router's setup screens or by browsing to http://whatsmyip.org/ from any system on your private LAN. You will need a daemon, such as sshd, running on the system you wish to SSH to. This seems to be working since you can SSH to the system from another system on the private LAN. Finally, you need to tell the router that incoming SSH requests should be sent to the desktop system. This is called port forwarding. The standard SSH port is 22, so you need to setup port forwarding for port 22 to your server's private IP address.
See, it's not so simple and you probably still have questions. Holler back if you need more answers or help configuring your router.