Marx's work is widely available. A good online source is www.marxists,org, which has all of the work of Marx and Engels, along with that by many other Marxists and writers relevant to a study of Marxism. The following excerpts come from Marx's early writings.
- Marx, Karl. "Estranged Labour." Economic and Philosophic Manuscripts of 1844. Karl Marx. www.marxists.org/archive/
- Marx, Karl. "Proceedings of the Sixth Rhine Province Assembly. Third Article. Debates on the Law on Thefts of Wood." Articles by Marx and Engels in the Rheinisches Zeitung (1842-3). www.marxists.org/archive/
- Consider these articles as documents giving some insight into significant formative experiences for Marx as he took part in efforts by the press to gain the right to cover legislative debates and as he closely observed how changing economic interests drove the transformation of legal prerogatives, expropriating some and empowering others.
If you haven't read much of Marx, start with The Communist Manifesto. The section on "Estranged Labor" comes from the Economic and Philosophic Manuscripts of 1844, early work not published until the 1930s. The German Ideology is another early work worth reading in thinking about the formative influences in the emergence of the historical Marx. The Marx-Engels Readers, edited by Robert C. Tucker, (2nd ed., W.W. Norton, 1978) is still an excellent anthology.