Games Workshop offers a free pdf of its Warhammer 40K battle primer, which contains basic for moving, shooting, fighting, using psychic powers and morale, along with other material to get you started.
40K
Genestealer Cult and Space Marine Chapters For 1 Page 40K
One Page 40K is a simplified version of the seminal miniatures wargame. The rules now offes data for Space Marine Chapters and Genestealer Cults.