You will need to know your computer's IP address to the outside world, which you can get by visiting
http://www.whatismyip.com - once you know your IP address, you should be able to go to any computer outside your network, and type that IP address into the web browser's URL box, along with the port number.
However, depending on your internet service, your IP could easily change on a regular basis, so you should sign up for a dynamic DNS service and get a free subdomain, where you can install a program on your PC, and then something along the lines of <yoursubdomain>.<yourdnsservice>.com will always point to your computer, so you wont have to know the IP any longer.
DynDNS is one of the best free services available, and I highly recommend it.