you need to install the nvidia driver. Ctrl+Alt+F2 to get to a TTY. Log in on the terminal. I forget what the user name and password is on the live image but should be easy to find here. Then install the nvidia driver:
sudo eopkg it nvidia-glx-driver-current
Once it's installed then restart lightdm
sudo systemctl restart lightdm.service
One that's restarted then you should be able to startx,
startx
This is from memory so someone feel free to correct this if it's wrong. By the way, if this is on the install then you may have to reexecute this after the install is completed and you boot into the newly installed system. if that's the case, when you get to the terminal screen, use your username and password and not the live media one.
