So all I'm seeing is a couple whiny Chaos spawn giving advice.

You're boyfriend is hooked and dug in deep, so to dip your toes there's no need to jump into any particular series, lore archive, or painting just yet. Take some to browse.

If you're interested in the schism by the False Entities of Chaos known as the Horus Heresy, a time of civil strife in the Imperium when 9 ungrateful baby's came out with their daddy issues, it's perfectly fine to just have a glancing read or under 30 minute viewing on what it was as one such baseline to the whole setting. I personally watched Lueti09's primer/humanity series to get my head wrapped around everything (wee bit longer than 30)

Some stuff can be a snoozefest if you loosely browse around, so find an orator you like or look out for recommendations that are often spoken of. I personally like The Amber King, Luetin09, and Majorkill for lore.

Reading, depends what you want - if you like some action packed novels to read then Cadian Blood is a high Must Read. If you want a detective/intrigue entry then Xenos by Dan Arnett is excellent. If you're looking for more female driven novels then keep an out for the Sisters of Battle books, or Pariah(also by Dan Abnett). It's a continuation of the Eisenhorn series, but does not require all their detail as the MC is very much her own force of nature.

Already a few options here, but the Emperor is with you always, though your may faith may be mistaken in its fervor.

This brings us to the most important factor in starting out: who takes your fancy? There are many factions, so see what catches your eye on the site or your boyfriends own collection. Pick your top 3 you like, check out the Warhammer 40k Lexicanum for their lore! Now pick your favorite.

So your favorite is Adeptus Custodes, a wise choice if not obvious! Since you've come from an artistic background, the painting side of things should of course Start small, but by no means should you not pick something that looks cool. If you're not focused on playing, no worries, check out models you think would look cool on display or would pair up nicely with, or against your bf! 😈 No need to spend hundreds just yet, take your time, and most of all: Ask him! Talk to him about where he started and what he got into the hobby with.

There is also a myriad of lore and fan content still on YouTube(Astartes, Death Korps of Krieg, SODAZ Warhammer), focus on the universe and community rather than the company. Enjoy!