What to read
Immanuel Kant. Groundwork of the Metaphysics of Morals. 1785. (Translated by Mary Gregor, Cambridge: Cambridge University Press, 1997).
Read: Section I (“Transition from common rational to philosophic moral cognition”), pp. 7-18.
What to look for
Kant begins the Groundwork with what he takes to be a commonsense belief concerning the special value of acting from the motive of duty. The motive of duty involves doing something simply because it is one’s duty, the commandment of the moral law.
The rest of the book consists in an attempt to show what must be the case for there to be such a motive.
Hume agrees that there are instances where people are motivated solely by the thought that it is their duty. See Treatise 3.2.1.1-8, pp. 307-9; 3.2.5.5-6, pp. 332-3. But he does not treat these as the central cases of moral behavior. On the contrary, it seems to me, he thinks of them as strange, cases that require special explanation.