You need to do two things:
1) get ports forwarded by your router
2) find your IP address
The first thing you need to do is log in to your router's control panel. Open a web browser. The address of your router depends on what model it is, so google it if you're not sure. Some common ones are 192.168.1.1 and 192.168.100.1. Log in as the administrator (again, the default username and password vary, so google your router to find out, but they're usually either blank/admin, admin/blank, admin/admin or something like that).
Go to the Port Forwarding page and set the ports your server is running on (see below) to your server's local IP (192.168.x.x).
I'm not sure exactly what kind of filesharing you're doing, but AFP is on port 548 and FTP is on 21. If you're using some other protocol, google to find the port.
Next you need to find your external IP. The easiest way for most people to do this is to go to
http://showmyip.com/ or some similar site.
Now, when you connect to your server from home, instead of typing in 192.168.x.x, type in your external IP to make sure the connection works.
If it does, you should be able to use that address to connect from anywhere in the world.
If you don't have a static IP (which you probably don't), I suggest using a service such as DynDNS so that you will have a static (and for most people more memorable) address to connect to. Visit
http://www.dyndns.com/ for details.
Hope this helps.