The topic, "Karl Marx and Communism" is rich with possibilities for argument. You should read some of Marx's writings and some books written about him and about Communism. That will make your task much easier. Here are some suggestions.
A thesis is an idea which you want to test against evidence. It's similar to a hypothesis in the natural and social sciences. Most people formulate a thesis as a question they want to answer or a problem they want to untangle. Ask yourself, "What is it about Karl Marx and Communism I find interesting, challenging or unusual, and how can I explain that?"
Of course, this means reading. Read as much as you have time for. A lot of Marx's economic and philosophical texts are dense and difficult, but they will reward careful study. Start with easy editions, abridged or annotated editions of his most famous works. Capital, Grundrisse, The German Ideology and The Communist Manifesto are all available with helpful notes and commentary. "Collected works" editions will also give you some of his best stuff, like "The Eighteenth Brumaire of Louis Bonaparte" and his letters.
Once you've got a background in Marx, you might also read up on Communism from other authors. Vladimir Lenin wrote a lot about how to make Communism work in the real world. There are great books on The Paris Commune. There are a lot of Twentieth Century authors, from Bertrand Russel to Jurgen Habermas to Mao Tse Tung, who wrote about Communism or tried to implement Marx's ideas and wrote about that.
The more you read, the more thesis statements will suggest themselves. If you're still struggling, think about problematic connections. What did Marx really think of the working classes? You can find the answer to that in the writings of his colleague Friedrich Engels, but then you'd have to connect it to Marx's own writing. Which is the better explanation of economic development, Marx's Labor Theory of Value or David Ricardo's Theory of Comparative Advantage? The answer is "both," but you could explain why. Is Communism possible in only one country, or must it be a global phenomenon to really work? Leon Trotsky struggled with that one.
If you read first and write later, you'll have a much better grasp of your task and the possibilities. You've chosen, or have been given, a massively complicated and potentially great topic. Proceed wisely and you will be richly rewarded. Make good use of the library. Good luck!