The best education would come from an Owner’s Manual, but if you are asking these questions, I suppose that the car didn’t come with a manual.
That being said, the ECT button delays the shift points for the transmission, so that the engine’s RPMs build up to a higher point, thus allowing slightly faster acceleration. This will definitely reduce your gas mileage and may produce a “hard” shift.
Cruise control has the potential to improve your gas mileage if you are driving on a highway where you can maintain a steady speed. If traffic is heavy/congested or if it is raining, do NOT use the Cruise Control.
The Overdrive control allows the transmission to shift to its highest gear (most likely 4th gear) when the car gets to a steady speed of…about…35 mph, and this will improve your gas mileage. If you are driving in an urban environment, I suppose that you could turn off the Overdrive, as the transmission wouldn’t be able to shift into its highest gear under those circumstances, but…you would probably forget to turn the Overdrive back on later, so I would advise keeping the Overdrive turned on at all times.