For a simple five-paragraph essay, forgiveness can be an unwieldy concept. It’s the same as writing an essay on the meaning of love: impossible to do well in less than five hundred words, unless you connect forgiveness to a very tangible, concrete situation.
Have you recently read any novels (for school or outside of school) that connect to the theme of forgiveness? If your high school or middle school reads The Kite Runner, you could connect Amir’s guilt over his failure to protect Hassan from violence, and its repercussions twenty years later. If your school reads any memoirs, like Marjane Satrapi’s Persepolis or Elie Wiesel’s Night, you could discuss how each work's author/protagonist forgives/doesn’t forgive their persecutors.
Have you thought about writing a thesis statement on a current issue? For example, you could easily research the idea of forgiveness as it relates to debates over the death penalty, or in an even more recent current event, the forgiveness extended by the family members of those murdered in the Charleston shooting.
Start with this link to NPR to research the death penalty debate. http://www.npr.org/2015/04/22/400445794/debate-is-it-time-to-abolish-the-death-penalty
The problem with writing an essay on forgiveness is that it can very easily degenerate into a meaningless cliche. It is very easy to say "I forgive you", and saying that makes the person uttering the phrase appear magnanimous, but like public apologies, it can often be more a form of public display of one's own claims to moral merit than something really benefiting the person being addressed.
Since you listed this under religion, one way to start you paper would be with an important text from the New Testament, Matthew 6:6:
But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.
You could use this to develop a thesis concerning the difference between forgiveness as a public performance and forgiveness as a private act.
You could write a thesis statement along the lines of "While forgiveness is now often a public performance, intended almost as a form of self-aggrandizement, it has a long history in the Christian tradition as a ritual act, often performed as part of a cycle of atonement and reconciliation with God and neighbor during Lent as part of preparation for Easter communion."
Ephesians 4:32 - And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you.
"Without forgiveness life is governed by...and endless cycle of resentment and retaliation." - Roberto Assagioli
Forgiveness is not necessarily giving your approval. It is just a way to cover and forget the offense in order to have peace. Everyone makes mistakes. We are all sinners. If we held a grudge because of every wrongdoing, the world would be just as Roberto Assagioli described life without forgiveness.