I would imagine you need to edit the config.txt file so that you have the HDMI mode set to VGA.

So…

sudo nano /boot/config.txt

Scroll down to the bit that says

hdmi_group=1

If I were changing it, I would put mine something like this:

hdmi_group=2
hdmi_mode=35

This would set the HDMI group to DMT and the resolution to 1280×1024 @ 60Hz.

Then finally:

sudo reboot

You might need to play around with the hdmi_mode to one that works for your monitor / TV. And if you are using a TV, then it might be easier to set the HDMI group to 1 and then pick a HDMI mode that your TV is capable of displaying (I would hazard a guess as hdmi_group=1 then hdmi_mode=4 (which is 720p) or mode=16 (which is 1080p).

You’ll find a lot of information here if this helps: http://elinux.org/RPiconfig#Video