Before anything; READ THE FICTION PRIMER!!
I know, reading is boring, but it fleshes out the game so much it’s almost a shame to pay for it and not know the lore.
Here’s a little hint to get you started: Humanity never discovered FTL-travel, yet we’ve conquered the galaxy…
What makes Rimworld so good, for me personally, is the depth it has.
I know of no other games where your colonists can have fingers shot off, broken wrists, etc.
I also know of no other game in which you can harvest organs to implant if your colonist has an old heart!
Diseases, psychological breakdowns, a nice social system, …
It all adds up to very immersive gameplay, which I happen to like.
It has a large community and a lot of mods, so browse through the workshop a few times in the next few days to add some stuff that you might find usefull to reduce the micromanagement a bit or to add buildings/features. Or visit the nexus(but, since there’s a workshop, I’m more lazy than impatient about mods so I’ll just browse daily).