An active voice is a straight out sentence like "She wrote a song". The focus is on the she and that she wrote a song. Simple.
Now the passive voice is different. This is a tip that I use a lot to figure out the difference. Passive voices usually have a to be verb like is, were, was, and a past participle, like learned, cleaned, and watched.
In this case, the sentence in passive voice would be "A song was written by her." Notice how was is a to be verb and written is a part participle.
Hope this clears up any confusion and write in the comments if you have any questions. And good luck on your test :)
