At the top of the inbox, click the blue settings (
) button,
then on the settings page, go to the "Preferences" link under "Personalization" in the left settings column,
then find where it says "Do you want to always compose messages using the graphical editor?" and un-check "By default, compose messages using the graphical editor",
Then click the blue "OK" (
) button at the bottom of that settings page.
Then it will compose messages with the plain editor by default, and at the top of that editor it will give you a link to switch back to "HTML Editor".