If you want to read some 40k stuff without needing to know a lot of backstory and would like something considerably lighter than the "grimdarker than black" that the setting is known for, I recommend Ciaphas Cain. It's a comparatively lighthearted series about a cowardly commissar who by incredible luck and/or subconscious skill, against his own efforts, happens into situations where he becomes more and more of a hero of the Imperium.