I just got a vps server running 12.04, I have ssh access but I don't know how access GUI and remote desktop
Usually VPS don't have a desktop environment, so you have to install one from the
ssh connection, install VNC and after that you'll be able to use a VNC client to connect to your remote desktop.
Just keep in mind that graphical data is far more bandwidth clogging than text. I.E. VNC will use more bandwidth than just browsing the web. Plus any media content on sites will not be very smooth on VNC, if they play at all.
And your desktop environment will use a lot of memory of your VPS, so in short in general is not a good idea to have a VPS with a DE, but if you really need it connect via ssh and run:
apt-get install ubuntu-desktop gdm vnc4server