A neat starting point is Engel’s Socialism: Utopian and Scientific in order to see how Marx and Engels’ conception of socialism differs from previous utopian conceptions of socialism, mainly that theirs is scientific. Also, Lenin’s The State and Revolution is a must-read in order to be able to catch Kautskyist, opportunist, distorted interpretations of Marx and see what Marx really meant.

I’ve also found that Stalin’s Anarchism or Socialism? and his interview with H.G. Wells do a really good job at explaining the flaws with liberalism.

It also wouldn’t hurt to read Politzer’s Elementary Principles of Philosophy in order to get a better understanding of the underlying philosophy of Marxism: dialectical materialism. It also serves as an easy criticism of metaphysics and idealism.

For a compiled study guide with lots of books that will teach you more about communism, refer to Dessaline’s Marxism Study Plan.