Luetin's introduction to 40K, found HERE, is a brief 17 min overview of the game origin and basic premise. You should watch it. He also has a 3 hour synopsis about the rise of the God-Emperor and the foundation of the Imperium in two parts, HERE and HERE, that is well worth the time it takes to watch.
I'm also a big fan of the Oculus Imperia channel, and his video on The Unification Wars and the Mechanicum of Mars provide damn good insight on two critical aspects of Imperial history.
THE WIKI'S:
Lexicanum - This will be my primary wiki source for links.
40K Wikia – Good info, some of it explained even better than the Lexicanum, but plagued with link ads. Browse at your own peril.
1d4Chan – The place that gives the info, with a healthy side of sass and attitude.
THINGS TO KNOW:
1. The Warp. An alternate dimension which is the source of both FTL travel and an endless swarm of demonic creatures bent on destruction of…well…everything.
2. The Imperium of Man. The galactic government bent on domination/liberation of the galaxy to promote the survival and interests of mankind.
5. Psykers. Mutants who are totally not wizards. They channel the Warp to create alter the rules of the physics and travel through the Warp. They get possessed by daemons a lot, so no one likes them or trusts them even though you can't travel the Warp without them.
6. Daemons and the Ruinous Powers. Beings born of the Warp, and the inter-dimensional entities they serve.
7. Xenos. The other sentient (sort of) life forms which populate the galaxy. They're totally not analogous to fantasy races like ELVES, UNDEAD, and ORKS. No, wait, they're still orks. In space. With guns. Yeah. There's also TYRANIDS (read: xenomorphs), but not DWARVES (because the 'Nids ate them).
8. The Inquisition. +++REDACTED+++
9. Rogue Traders. Ever wonder what it would be like if you were a Spanish conquistador flying around space with unlimited authority and a galleon that shit cannon balls the size of a meteor? That's these folks.
10. The God-Emperor and the Imperial Cult. A dead psyker wired into a life support system, alternatively considered a God whose Golden Throne sustains him in his slow process of rebirth. The cult is the creed of fundamentalist extremism/glorious enlightenment that promotes His worship.
EVENTS TO KNOW:
1. The Dark Age of Technology. We were like the Star Trek Federation once upon a time. Exploration, peaceful relations with other species, AI, androids servants we called "the men of iron", and everything was looking good.
2. The Age of Strife. Shit got real, and the wheels came off the bus.
3. The Age of the Imperium. Space Jesus led us back out to reconquer the galaxy, with a flaming sword in his hand and payback on his mind. He got betrayed by his favorite son, shenanigans ensued, and everything is "grimdark" now.
I'm also a big fan of the Oculus Imperia channel, and his video on The Unification Wars and the Mechanicum of Mars provide damn good insight on two critical aspects of Imperial history.
THE WIKI'S:
Lexicanum - This will be my primary wiki source for links.
40K Wikia – Good info, some of it explained even better than the Lexicanum, but plagued with link ads. Browse at your own peril.
1d4Chan – The place that gives the info, with a healthy side of sass and attitude.
THINGS TO KNOW:
1. The Warp. An alternate dimension which is the source of both FTL travel and an endless swarm of demonic creatures bent on destruction of…well…everything.
2. The Imperium of Man. The galactic government bent on domination/liberation of the galaxy to promote the survival and interests of mankind.
5. Psykers. Mutants who are totally not wizards. They channel the Warp to create alter the rules of the physics and travel through the Warp. They get possessed by daemons a lot, so no one likes them or trusts them even though you can't travel the Warp without them.
6. Daemons and the Ruinous Powers. Beings born of the Warp, and the inter-dimensional entities they serve.
7. Xenos. The other sentient (sort of) life forms which populate the galaxy. They're totally not analogous to fantasy races like ELVES, UNDEAD, and ORKS. No, wait, they're still orks. In space. With guns. Yeah. There's also TYRANIDS (read: xenomorphs), but not DWARVES (because the 'Nids ate them).
8. The Inquisition. +++REDACTED+++
9. Rogue Traders. Ever wonder what it would be like if you were a Spanish conquistador flying around space with unlimited authority and a galleon that shit cannon balls the size of a meteor? That's these folks.
10. The God-Emperor and the Imperial Cult. A dead psyker wired into a life support system, alternatively considered a God whose Golden Throne sustains him in his slow process of rebirth. The cult is the creed of fundamentalist extremism/glorious enlightenment that promotes His worship.
EVENTS TO KNOW:
1. The Dark Age of Technology. We were like the Star Trek Federation once upon a time. Exploration, peaceful relations with other species, AI, androids servants we called "the men of iron", and everything was looking good.
2. The Age of Strife. Shit got real, and the wheels came off the bus.
3. The Age of the Imperium. Space Jesus led us back out to reconquer the galaxy, with a flaming sword in his hand and payback on his mind. He got betrayed by his favorite son, shenanigans ensued, and everything is "grimdark" now.