Communist Manifesto:
https://www.marxists.org/archive/marx/works/1848/communist-manifesto/
Value, Price, and Profit:
https://www.marxists.org/archive/marx/works/1865/value-price-profit/index.htm
Capital:Chapter I:
https://www.marxists.org/archive/marx/works/1867-c1/ch01.htm
Critique of the Gotha Programme:
https://www.marxists.org/archive/marx/works/1875/gotha/
Draft of a Communist Confession of Faith:
https://www.marxists.org/archive/marx/works/1847/06/09.htm
The Principles of Communism:
https://www.marxists.org/archive/marx/works/1847/11/prin-com.htm
The Myths about Marxism:
http://www.marxist.com/the-myths-about-marxism.htm