Right, first of all go to the Dash (the thing in the top left of the screen) and type
desktop sharing. Click on the
Desktop Sharingapp to start it. In its preferences make the settings look like:
Close, and your computer should now be ready for someone else to connect. The thing is that they need to know the address to connect to. To find this out, do a Google search for
what is my ip addressand it'll come up with your public IP address. Something like:
Tell the IP address to the person you want to share your desktop with. Ubuntu will prompt you to allow / refuse when someone tries to connect. Note that your IP address may change every now and then. You can use a host name that doesn't change using something like NoIP.