For me, my stories start with a character and a simple element after which I ask "what if" to just about every aspect my imagination can create from those. For my first book, it was a character who felt like an outcast. My second novel, Gone Before Dawn, grew from a nightmare I'd had and was fed by research I did of my local area. Each element of it, the graveyard, the haunted apple tree, the mansion, all of it came from places I knew around my local area.
The Manipulated Evil Trilogy came about after 9/11 and what followed. I felt helpless which gave rise to a character who was anything but and a story that grew into something much bigger than I had ever planned. The jumping off point for Rise of the Arcadians was the dynamics of a big family inside global climate change and how cycles might change the world from what we know today. Among the Ancients started to form as I was researching ancient mysteries - like how did they move the immense stones that make up Stone Henge or the walls of Sacsayhuaman - and what would a man do if he was suddenly face to face with the answers? Or possible answers. :-)
The whole of the seven book Disillusionment Series grew from my constantly asking the question "what if" while researching the beliefs of the ancient Sumerians.
My next book, well, the idea for it started from something my middle daughter once said. A story where the bad guy might be good or the good guy might be bad or... well, you get the point. Pieces of other things are also filtering in.
Ideas can come from personal experiences, the morning newspaper or news report, research of the past, movies, other books, or anything at all. An author's job is to people the idea with unique and complete characters to bring it all to life and keep it as unpredictable as possible. The stronger the characters, the easier that is to do. So, I suppose, knowing a bit about human psychology helps a lot with that. Each personality will react differently to any given circumstance, and there is where the fun really begins...