We use active voice most of the time. This is where the agent, or 'doer', comes before the verb. For example:

• Active voice
  We will close your account
• Passive voice
  Your account will be closed

Sometimes, the passive voice can make your content read more naturally, so there's no need to avoid it every time. It can also be used in error messages caused by user error, as it's less accusatory.  For example, "your password was entered incorrectly" is in the passive voice, but it's friendlier and less accusatory than "you entered an incorrect password".

A good rule of thumb to help you identify passive voice is to add the phrase "by monkeys" to the end of your clause ("Your account will be closed... by monkeys"). If it still makes sense, you're probably writing in the passive voice.