Your Energy refills automatically and periodically according to your Level in the game.
Note that you will receive a one-time award of 10 Diamonds and 50 Energy units when you connect to Facebook in the game for the first time.
Can get more energy by tapping the '+' sign at the top of the game screen and exchanging Diamonds. And you can buy Diamonds to exchange for Energy.
When you buy Diamonds, consider the 'Restore and Increase' option as it is very powerful! It increases your capacity for Energy and fills it to that new capacity.