Had better refers to the immediate future. It is followed by an infinitive without to. It is a strong advice. We use it to tell people what to do.
- You had better consult a doctor.
- You had better apologize.
- We had better hurry up. We are already late.
Had better sometimes suggests a threat and is not used in polite requests.
- You had better tell them soon. If you don't, there will be trouble.
- You had better mend your ways.
To make negative forms, we put not between had better and infinitive.
- You had better not irritate him.