Let's fix your question and statement first.
I am studying about the passive voice. Can anyone tell me anything about it? When must we use it?
If I made a mistake, please let me know. My English is not very good.
Now, about passive voice. One thing to keep in mind is that good writers recommend that you don't use it very often, only when you really need to. Active voice is almost always better, because it's more direct and usually shorter.
I can't offer a "rule" about when you should use it. It's generally used when you want to emphasize the object of the action rather than the subject.
So if you write, "the book Miguel was reading was red." you focus on the book. If you write, "Miguel was reading the red book." you focus on Miguel.
Do you have some specific sentences in mind?