If you are writing even slightly outside your personal knowledge base, then there is nothing more important than research. But be careful! It can suck you in and crush you destroying your story.
I have done research for probably 90% of the stories and novels I have written. I want to be sure of things, make them feel real. I want to casually mention the weird, whirring, things that my astronomer is using in a steampunk story. I want to have those little inside jokes that only people who work in a certain job know. This is easier than ever with the plethora of resources we have. But that also means you need to watch how deep you go...
My personal favorites to start with are Reddit, Youtube, and a good old fashioned google search. It really depends on what I am writing as to where I go from there. For example, I watched hours of youtube while writing a nuclear fallout story. I learned about Geiger counters and the little cracking sound they make. It was a short story so I didn't need to know too much. My characters are always the focus of my story but I want the setting to be second only to them. There is some grace in fiction though. So long as you are not writing something obviously unbelievable, you can get away with a few incorrect or flourished details. But you want it to feel as real as possible.
From these sources I'll get keywords, ideas, instruments, sayings, jargon and then search those. Some people stop after just a quick google search and that's probably not the best idea (but it does depend on what you were goolging, sometimes you just really need to know the line up of a football team and that can be fast). After I get more in-depth with the things listed above, I hit up harder, more precise sources if I need to. Then, I go to the library. I know. How pretentious am I? Sometimes, you have to go the whole nine yards...I get books, documentaries, and find articles in specific magazines if I need to. Research doesn't always go this deep. Sometimes I don't have time or the subject doesn't need that much info. But I do this so I am fluent, or my character can pass as fluent.
If I am writing about the American Civil War, then I might watch movies to get a feel. Look at the clothing, read the history, get to know the politics, the food, what songs they sang. I do all this just to drop in a quick line about it. Something a reader might never pick up on, or care about. But it feels true.
I also want to warn against getting too deep into research when you don't have to. I am guilty of this as well, but I know in the online writing community, how much research and how weird your research is, is a HUGE flex. Sometimes it's just fun and it can be innocent to talk about the weird stuff you are finding. But make sure it is necessary. Sorry to side track. Don't get sucked in and never come out. And probably don't (I love you, Michael Crichton) use too much jargon to where your reader is no longer an interested bystander but is now rolling their eyes at your pretension. Writers are super pretentious...Don't talk down to your audience and don't drown them in technical jargon. This is a novel, not "How To Successfully Splice Human and Animal DNA for Advanced Fanatics, 207th Edition". Don't do so much research that you forget to write the novel! :)