We use "have to" for things that are required.  It is similar to "must".  (have to + infinitive)

For example: "Mariam, you have to take the exam before you can go to term 7."

Huda has no choice. She is required to take the exam.

Another example: "You have to get at least a "3" on your PDA for to pass the semester.

So, at least a "3" is required to pass the term.

We use "don't have to" to show that something is NOT required.

This means you can choose to do the action or not do the action.

For example: "The homework is on page 42 in your work book. You have to do exercise number 3, but you don't have to do number 4. " (You can do number 4 if you want to, but it is not required)

Should:   (should + infinitive)

We recommend, or suggest what we think is a good idea for them to do. The person does not have to do what we say.

For example: You should go to sleep early on the night before a test. This is only suggested. You are not required to do this; the police will NOT come to your house to check!

We use shouldn't when we think something is a bad idea.

For example: "Hind, your grades are not very high this term. You should study more."

For example: "Your grades are not very high this term. You shouldn't sleep in class!"

Other expressions to offer advice are:

"I suggest that... I would advise you to..., I recommend that you... If I were you I would..."