I'll start by giving some examples.See if it helps.
Somebody cleans the room every day. (active)
The room is cleaned every day. (passive)
Somebody cleaned the room yesterday. (active)
The room was cleaned yesterday. (passive)
In many situations, there is an agent(the person or thing who does something) and a recipient (the person or thing that something is done to).
If we want to make the agent the subject, we can usually do this by choosing an active verb form.
If we want to make the recipient the subject, we can usually do this by choosing a passive verb form.
That should be enough to begin with.