Need help with on off switch to power arduino.
Since you are using an Arduino Uno board, putting the MCU (the ATmega328P-PU) to sleep won't save you much battery juice. Putting the MCU to sleep won't make a difference because the voltage regulator on the Uno will waste a lot more power than anything else on the board. The ON LED will also draw significant amount of current on that board.

To really save power you should look into getting an Arduino board that has a more efficient voltage regulator, or use no regulator at all by changing your 9V battery by, say, 4 NiMH AA cells.

Here's some more information about the issue for you to look at: What are (or how do I use) the power saving options of the Arduino to extend battery life?