First, to try out the edit, simply hit the e key when you are at the GRUB menu at the proper entry, then edit the line with vmlinuz close to the beginning, adding nomodeset at the end. If this works, you need to edit the /etc/default/grub (as root) to add to the end of the line as follows:
The original top of the file looks like this:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset"
# If you change this file, run 'update-grub' afterwards to update
# For full documentation of the options in this file, see:
# info -f grub -n 'Simple configuration'
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
# Uncomment to enable BadRAM filtering, modify to suit your needs