Use could, should, ought to, and had better to give a suggestion or advice to someone.
Could is used to show an option.
You could go to the party later or you could go to bed early.
Should is used to show something that the speaker thinks is a better option.
You should probably go to bed early. You look very tired.
Ought to is used to show a strong suggestion. The speaker really thinks this is the best option for you.
You are not getting good grades in class. You ought to pay better attention in class.
Had better is used to show that the speaker thinks the suggestion is very important.
You had better do what your boss told you to do or you could get fired!
Use must or need to to show that something is necessary.
You must eat food or you will die.
You need to go to a doctor because you look quite ill!