The original versions of Marxist classics remain the best way to study Marxism. It is even better when groups study together.
For a quick look at basic concepts, though, these notes are offered.
Origin of the Family, Private Property and the State
Socialism, Utopian and Scientific
Imperialism, the Highest Stage of Capitalism
The ThreateningCatastrophe and How to Fight It
Revolutionary Education and the Works of Paolo Freire
The Proletarian Revolution and the Renegade Kautsky
Fundamental Problems of Marxism (Plekhanov)
The Foundations of Leninism (Stalin)