This (scroll down halfway to the economic doctrines section) talks about different economic schools including the petty-bourgeois views of Sismondi and others. You should also read the Principles of Communism.
ProleWiki also has two beginner reading lists for Marxism: 1 2
I would recommend reading a lot of other theory before starting Kapital. The USSR also wrote a textbook on Political Economy that was designed for students and may be easier to understand. It also talks about things that aren’t in Kapital like imperialism, socialism, and feudal and slave societies.