One Page 40K is a simplified version of the seminal miniatures wargame. The rules now offes data for Space Marine Chapters and Genestealer Cults.
40K
Warhammer 40K Battle Primer
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.