We can use 'should' to make a recommendation or to give advice.
You should visit Paris while you are in France.
I recommend you visit Paris. I think Paris is a good place for you to visit.
Children should drink milk for breakfast.
I recommend children drink milk for breakfast. I think it is a good idea for children to drink milk for breakfast.
We can use 'should' to express obligation. 'Should' expresses a weaker obligation than 'must' or 'have to'.
I should go, or I will be late.
I have to go. If I don't go now, I will be late.
'Should' is often used to express an obligation that the speaker does not intend to meet.
I should finish my homework, but I'm too tired.
I know it is important to finish my homework. I'm not going to finish my homework.
We can use 'should' to talk about something we expect or something that we think is likely to happen.
They should be here at 6 o'clock.
I expect them to arrive at 6 o'clock.
$10 should be enough to buy a sandwich.
It is likely you will be able to buy a sandwich with $10. It is not certain.
The short form of 'should not' is shouldn't.