We use 'should' for giving advice.
You should speak to him about it.
You should see a doctor.
You should ask a lawyer.
We use 'should' to give an opinion or a recommendation.
We should invest more in China.
They should do something about this terrible train service.
He should resign.
'Should' expresses a personal opinion and is much weaker and more personal than 'must' or 'have to'. It is often introduced by ' I think'.
I think they should replace him.
I don't think they should keep the contract.
Do you think I should tell her?