Without knowing your exposure, the simplest, fastest, and most straightforward primer is How Marxism Works, by Chris Harman. If you have any objections to Marxism, you likely won’t find answers in it though, as it is extremely brief. Additionally, Harman’s views on feminism are outdated, believing Marxism to supercede feminism, rather than the two implementing each other.

Otherwise, The Principles of Communism, Socialism: Utopian and Scientific, Wage Labor and Capital, Value, Price and Profit, Critique of the Gotha Programme, and finally Manifesto of the Communist Party is the best order of the essential Marxist works to understand the majority of the basics. These will walk you through terms, then Dialectical and Historical Materialism (the philosophical side of Marxism and failures of non-Marxist Socialists), 2 brief works on Capitalist critique, a critique of a weaker reformist Socialist Program, and finally a call to action, tying it all together.