You have Energy up to a rating of 12 when you start the game. Once it is all used up, it recharges at a rate of 1 Energy every 5 minutes. As you progress through the game the capacity of your energy increases from the 12 you are originally given as you level up. You are given 1 extra to your maximum Energy each time you level up.
Energy is required for building, harvesting, visiting Neighbors, and in fact, pretty much every activity in CityVille (except planting crops, which you are permitted to do for free). Whenever you undertake a task, you lose 1 Energy. Harvesting crops costs 1 Energy, as does pulling up dead crops. Buildings require more Energy to fully construct, so be sure to inspect this rating before undertaking any construction work. You will need to click on the construction site multiple times to keep them building - using 1 Energy with each click.
You can see the amount of Energy that you currently have, indicated by the bar in the center of the screen. Hover over it to see your maximum Energy level. If you run out of Energy, you will either have to wait 5 minutes for one piece to recharge, or use some of the other techniques for getting Energy quickly.
A good way to get more energy is to give Batteries to your Neighbors. This will create Energy for them and, if they return to favor for you, is a massive boost to your own energy supplies. Getting a network of hard-working, regular-gifting Neighbors is one of the best ways to make the most of your Energy.
If you click on a building and see a lightning icon, click on it, as this will give you instant Energy.
Spend your energy wisely - don't start too many construction projects at the same time, especially in the beginning stages of the game as you will find that you soon run out of energy and are stuck with half finished buildings. You'll also not be able to collect cash from your businesses etc... when you have zero energy, so spend wisely!
Here are some tips on gaining energy in CityVille.
1) Wait come back now and again.
Your energy bar is topped up by 1 every 5 minutes so this is a slow but sure way to get your energy levels back up.
2) Level up
Make sure you have depleted your energy stores just before/as you level up as levelling up will restore a big chunk of your energy bacr when you level up.
3) Visit friends
You can get an energy boost when visiting friends.
4) Energy batteries
If you have some cash you can get these energy batteries and utilise them yourself or start a sharing network with some friends to distribute these big energy boosters.
5) Get lucky
Yes sometimes general gameplay actions will result in you being given a boost of energy