you need to activate a vnc service on a particular screen... depending on which vnc server you have installed.. it should be something like:
in a terminal... then it should ask you to enter a password for the connection.. also note, that whatever username is logged in and started the vnc service, is the user that will be logged in over the vnc connection... hence, you probably don't want to do that as root... I'm not an expert with vnc by any means, but I've used it a little.. there is also a Java VNC you can setup through Apache, and then you can just access a webpage and it will load the desktop... kind of tricky as I understand it, but then you have the ability to use the VNC from any computer that supports java..
P.S. - this is probably better suited in the Software forum?