https://www.youtube.com/watch?v=JEVpo2QO6lsIf you have a controller plugged in when you start the game, it should work right away. (At least Xinput/XBox controllers will)
The gamepad UI usually has to be turned on in the settings, per character.
I like to set a key mapping to toggle between the controller and keyboard UIs - either control method will work regardless of what UI you have up. (ESO won't auto-switch between controller and keyboard UIs.)