You can goto this address and get your external IP.
http://www.whatismyip.com/
Next you need to setup port forward to forward certain address from the external IP to the internal lan IP. Docs from maker of the router should help.
Or on many routers you can define a DMZ which will allow one internal lan machine to be complete seen on the wan external side like it was actually the external IP. Not the smartest idea unless the machine you define is well protect with firewall rules and security patches applied to all running services.
To see if port is open goto here and use the advanced port scanning tool.
http://www.hackerwatch.org/probe/
Things you may run into.
Some providers block common server ports so you may have to do this. Start with high range ports.
Example forward port 22 sshd connection if port 22 is blocked.
External IP port 10001 forwarded to Internal IP port 22
So with this setup you use the external IP and port 10001 and it conects to lan IP port 22.
Other thing is testing portforwarding from with in your network. Many store bought routers lack the ability to use the external wan IP. So for instance you have a web server running with port 80.
You type the lan ip from with the lan network in a web browser and it comes up.
You type the wan IP from outside from a remote site in a web browser and it come up.
Now if you type the wan IP from with in your lan network it usually will not come up.
Brian