Change the grub boot time out.
First, open a terminal and run this command:
sudo -i scratch-text-editor /etc/default/grub
You will see something like this:
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_TIMEOUT number 10 (it was line 9 for me) to any number of seconds (on my main computer I have it set to 2 seconds).
I'd advise against 0 seconds, because if you ever need to access grub, you will struggle - 2 is a good amount to chose.