Main points in an essay are what you want to convey to the reader. For your specific topic, you could include:
1. Promises (promising to keep the secret)
2. Knowing the difference between what secrets to keep and what secrets need to be told (if the person is in danger you shouldn't keep the secret). Also, what bad and good things can come from telling.
3. How to know if you can trust a person with a secret.
Hope I helped!