Had better (as a recommendation or advice)
You had better unplug that toaster before you clean it, else you may get a bad shock.
Had better not (advice to not do something or face the consequence)
You had better not say anything about the accident.
For all these circumstances you can also use "am going to", "should do" etc.
You shouldn't say anything.
You'd better do what I say or else you will get into trouble.
When we use "had better", it implies a definite negative effect if the advice or recommendation is not followed. In other cases, if you don't follow the advice something bad may happen, but on the other hand, something good may also happen.
However, if "had better" is used, then if you don't follow the advice something bad will definitely happen.