Honestly, I'd start with Blackshirts and Reds, it's a history book that analyzes AES states critically and debunks modern anticommunism and anti-Marxism. Michael Parenti uses modern, non-Marxist language to appeal to liberals and leftists sympathetic to Marxism but unsure about it and AES. An alternative, or supplementary source, is the famous "Yellow Parenti" speech and/or Albert Einstein's Why Socialism?

The absolute best primer is Engels' The Principles of Communism, which I would follow with Politzer's Elementary Principles of Philosophy, and swing back to Engels for Socialism: Utopian and Scientific. By now, you should understand what Communism is, how Marxism diverges from older Utopian Socialism, and have a thorough understanding of Dialectical and Historical Materialism, the philosophical foundation of Marxism. DiaMat makes the rest of Marxist theory far easier to understand, as it is the basis of all that follows, including analysis of Capitalism and Imperialism, and why Socialism is what comes after Capitalism.

Now, you can read Wage Labor and Capital as well as Value, Price and Profit back to back for Marx to explain the Law of Value without yet delving all the way into Capital, and prep you for Lenin's works Imperialism, the Highest Stage of Capitalism and The State and Revolution, to bring you to the modern era of Imperialist Capitalism and explain the Marxist theory of the State.

If you read all of those works, you will be more well-read than the vast majority of Leftists. There are numerous other fantastic Marxist works, but this will get you by far the most bang for your buck when starting out.

As for Lemmy, the instances that care the most about Marxism are Hexbear.net, Lemmy.ml, and Lemmygrad.ml, both Hexbear and Grad have reading lists and offer help with theory.