There are a lot of things that you shouldn't tell your parents. Here are just a few examples:
1. Don't tell them about your plans to rebel or engage in risky behaviors. This includes things like drinking, drugs, sex, etc. If they found out, they would only worry and try to stop you from doing these things. It's better to keep this information to yourself.
2. Don't lie to them about important things. They will eventually find out and it will damage your relationship with them. Be honest with them instead so that they can trust you more.
3. Avoid sharing sensitive information with them that could be used against you later on. This includes things like your passwords, financial information, etc. Keep this type of information to yourself to avoid any potential problems down the road