Hi
Welcome to the android forums.
The back "button" is just to the right of the Home button. Its not a physical moving button, it reponds to touch.
If you do
menu button (to the left of home button)
system settings
display
scroll down to touch key light duration
you can make the buttons light up so you can see where they are. In the long term this unnecessarily uses battery, but may help if you are new to the device.
Roger