Reddit reviews The Thought of Karl Marx: An Introduction

The wikipedia page gives a good idea what Marxism actually is: A specific way to analyse societies, originally developed by Marx. Usually, the SEP is a much better resource for philosophical topics, but it's rather brief on Marx himself.
From there you can go into as much detail as you want, but as with any philosophical school of thought, it gets deep (and complicated) very quickly. You could just read an introductory text, his actual works with some companion pieces, or even go back to the philosophers that he drew from in developing his ideas.

David McLellan's commentary/anthology, The Thought of Karl Marx is arguably the best short one volume approach to Marx.

> This text provides the basic elements necessary for a grasp of the range and complexity of Marx's ideas. The first half of the book is a chronological account of Marx's ideas with a miniumum of biographical and historical detail. The second half is thematic and provides a concise summary of Marx's position, and extracts from his key texts on alienation, historical materialism, labour, class, the party, the state, revolution and future Communist society.

McLellan also wrote the most widely read biography of Karl Marx, and an all-you-need anthology.