Remote Ubuntu Screen sharing from Mac
In general terms, you'll need to:


Set up a VNC server on the Ubuntu machine. This is the software that shares the screen.
Set up a means of connecting remotely to the Ubuntu machine. If you can already SSH in, that's the best way, you can forward the VNC port over SSH by running this on your Mac: ssh username@ubuntuhostname -L 5900:localhost:5900, replacing the username and ubuntuhostname as appropriate.
Connect with VNC to the tunnel. You can do this with whatever client you like (including OS X's built in screen sharing), but the important part is that the hostname to connect to is localhost and you use port 5900.


If you need more specific instructions, I suggest you do a search, as most of these topics have been covered in some detail elsewhere. Ask Ubuntu is the best place to go if you need Ubuntu-specific information.