to give Access and Remote Control to an external user to your desktop, you should install x11vnc,
try this from your console:
$ apt-get install x11vnc
$ sudo x11vnc
from the remote vnc client put the remote ip and the display 0
that's it all!
There is another alternative if you can not execute the command (x11vnc) from the server for some reason, so open a remote ssh conection to the server (you have to install ssh server in the server side) and execute the command from the remote ssh client console (for example putty), if everything goes well, go to your vnc client and point to the server as I mentioned above.
When you execute the command on the remote console the port would be informed after the message feedback as you can on the following screenshot: