Read Socialism: Utopian and Scientific by Friedrich Engels. The quickest way to actually understand Capitalism is to work toward reading Marx's Capital with a group and other references. It might take a while to get there but this book by Engels will lay the groundwork for you to learn historical materialism and dialectical reasoning.

For Communism, read principles of Communism and the communist manifesto. But start with Socialism: Utopian & Scientific. Since communism hasn't ever existed, there have been communist groups and parties who may or may not have established different variations of socialism, there's nothing to study. You can study the history of communist parties and experiments, but there is a lot and much of it is sad or disappointing. Definitely learn about it, but its not where you should start.

Communism is a moneyless, stateless, classless society. Its a movement, something to work toward. It is not a system where the government owns everything. It isn't "authoritarianism". Its the opposite of those things.