Passive voice is considered one of the great evils of writing. However, there are times when it is appropriate, and even recommended, to use. Knowing when to use it is the mark of an experienced writer. First you have to understand what passive voice is. Think of it this way:
Passive voice: the subject (noun often) is receiving an action.
Active voice: the subject (noun, often) is performing an action.
A few examples:
Passive: The belles of the ball were gliding across the floor.
Active: The belles of the ball glided across the floor.
Passive: It is the intention of the belles to attract a husband.
Active: The belles intend to attract husbands.
The culprits are often the words: is, was, were, are. Seek these out and come up with ways to rework your sentence to make it more active. Think of the action happening now, rather than telling about the fact that it did happen. The time to use passive voice is when you are telling about something that already happened. But be careful, most action needs to be happening currently, even when you're telling in past tense third person. That is the tricky part.