Precisely which services do you wish to access from the "outside?" Printing services? File sharing services? SSH access? Web hosting? FTP?
If you have a typical home network setup with a router and some sort of cable or DSL service, then you're probably going to want to research "port forwarding." The short and skinny of it is that when you try and access your server from outside your local network, your router needs to know which computer on your local network to "forward" the traffic to. Each of the services listed above operates on a different network "port," and that port specific to that service needs to be "forwarded" to the local IP address (like 192.168.x.x or 10.0.x.x) of your server by your router.
So, two things need to happen:
1) Learn about port forwarding, and exactly how to set up forwarded ports with your specific model of router.
2) Learn about the specific services (FTP, mail, web, ssh, print, file sharing, etc.) you want to be able to access from the "outside" and which ports they operate on.
As a starter, ssh typically operates on port 22 TCP. FTP on 21 TCP. Web pages being served from your server would come on port 80 TCP.