Okay - write what I know. That's why I constantly talk about vampires, werewolves and mermaids.
Allow me to clarify, see when you hear the old adage, "write what you know" it doesn't mean what you might think.
Not clear? Let's try it a different way. Say I'm writing a scene where a vampire lurks in the shadows waiting to attack our innocent MC. Does that mean I have to have experience being stalked by vampires to do the scene justice? Let's hope not.
What it really means (in my humble opinion) is that I have to have knowledge of what the character might be feeling in this situation. A deep and all consuming fear? Powerlessness perhaps? Well, heck - I know those! I bet you do too. The situation I was in when I felt these things might be different from the one my MC is in, maybe not even half as dramatic. But the raw emotions are the same. So that's the experience I draw on. It's important to be able to generalize your experiences and feelings enough to recreate them in your manuscript. This may sound pretty simple, obvious even, but sometimes it's difficult to see what's right in front of your nose.
A second relevant interpretation of "writing what you know" has more to do with NOT writing what you DON'T know. In other words, I have full license to create an entire new world with my own laws and customs and people. But if I plan on writing about a member of the Supreme Court of the United States of America, I better either have first hand knowledge of law and the court system, or do my homework! Because no matter how artistically you can weave a story from your imagination, there will be others out there that see right through it because they DO know.
So do your research. Please. If you choose an actual location for your setting, and you haven't been there, find out as much as you can about it. Example: I've used Google maps street view. I've also pretended to be shopping for a house and searched online for information about certain neighborhoods and schools. And all for a tiny part of a single paranormal manuscript. But I wanted to make it as believable as possible. I wanted to feel like I was really there with my characters. Never did I take the attitude that since one of my characters was a demon, I didn't need to make the rest of it realistic.*
How can I expect readers to become invested in my story and characters if they can't put aside reason, at least long enough to fully immerse in my story and world? It has to feel real no matter how fantastic it is. And knowing what you're talking about makes that hurdle a lot easier to tackle.
*Side-note here - don't go overboard. Meaning, find out as much as you can to make the picture in your head complete when writing, but only put in the details that are important to the story and characters. It can be a tricky balance to maintain.