"Must" is used for something that is either
b) important or vital
c) a logical conclusion
or d) something you highly recommend
"Should" is used for something that is:
a) right or correct (a duty or obligation)
b) advice or a suggestion
c) something that is likely to happen
"Must" is stronger, and more emphatic. "Should" is not used for something compulsory, although it is for an obligation.
"Should" is more commonly used for "I should work now". But either can be used.