You're in luck Pete,
I've created a whole website stating how to do just that.http://www.myownserver.info
Probably the best place to start considering where you're at right now would be with port forwarding:http://tinyurl.com/portforwardingtutorial
I will just warn you now though, running your own public server poses a security risk and vulnerability to hackers.
I highly recommend using a PC that's separate and NOT your primary computer as your server. This way if your system does get hacked, you don't loose all of your important files, just the ones on your server. And if you manage it correctly, you should have regular backups of them and it's just a matter of recovering your server and restoring any lost files.
But beyond that, it's pretty simple and my website should be able to help you get started.