We use "should" to give advice, recommendations or talk about expectations and obligations we have.
For example: You should close your window, it is going to rain. ->recommendation
Paul should practise the new words for tomorrow's vocabulary test. -> advice
I should get home at 7pm. -> expectation
Should is followed by the infinitive like all modal verbs.
To negate a sentence use "should not" or shorten this to "shouldn't.
For example: He shouldn't come late tomorrow, we start on time!