Start off by reading Principles of Communism by Engels. You can find it on Marxists.org

There have been a couple of study guides linked around Lemmy and Lemmygrad, like this one. There’s also Dessalines’ Marxism Study Plan.

Some personal recommendations of mine would be:

• Silvia Federici’s Caliban and the Witch: Women, the Body and Primitive Accumulation
• George Politzer’s Elementary Principles of Philosophy
• Domenico Losurdo’s Stalin: The History and Critique of a Black Legend
• Mark Fisher’s Capitalist Realism
• Che Guevara’s Socialism and Man in Cuba
• Marx’s Critique of the Gotha Programme
• Marx’s The German Ideology

I listened to Communist Manifesto on audiobook. It’s good, but a bit short.

Marx’s Capital (das Kapital) is not so good for audiobook. It’s very involved and you really want it in front of you either in book form or a screen due to all the equations. Bit like a business/math textbook.