Did you choose to install the "Desktop environment" option during the second phase of the installation?
If not, run
and selecgt the "Desktop environment" option.
Alternatively, if you want to pick and choose in a bit more detail, you can do so too, e.g. if you want a lean approach, installing just x-window-system and e.g. fluxbox is a pretty reasonable choice.
If you have installed the GUI stuff, but it isn't working, did you choose the correct settings during the configuration of X?
To re-run the X configuration, execute
dpkg-reconfigure xserver-xfree86 --priority=medium
or if you want to get a bit more detailed questions, omit the --priority=medium
part of the above command.