theres should be a VNC server built in to OSX. Check sharing.
As far as accessing it from teh Intarweb, if you have a publicly addressable IP (not a 10.x.x.x, a 192.168.x.x, or a 172.16-32.x.x) you can just use your VNC client to connect to the address.
However, if you have a linksys or airport, or any other of the plethora of wireless access points/routers, you will need to proxy port 5900 connections to the mac's internal IP address...
I'd also highly and strenuously recommend learning about SSH tunnelling for VNC.