Received writing wisdom has it “write what you know.” Well, what if you are writing about three-limbed aliens or magical elves? How are you supposed to possess concrete, factual information about something fictitious? Assuming you are not simply adopting some other writer’s creation, how can you write what you know when you’re inventing it as you go?
Short answer? Write what you know around the elements created out of whole cloth. Your novel magical system has no existence outside the bounds of your imagination, but perhaps the characters employing it are based firmly on personality types you know only too well. Or the political structure of your secondary world is lifted directly from a term paper you wrote for your poli-sci course. Or the Space Legion’s battle tactics are exactly those you were taught in infantry school.
The shiny facade of your fiction may be something fundamentally unknowable, but the supports, the foundations, the unseen infrastructure holding the whole edifice together, making it read as plausible – these you should be familiar with. Inclusion of matters you have a comfortable working knowledge of will lend your story a sense of grounded verisimilitude that will encompass the fantastical elements as well.