Pinterest can be a surprisingly helpful resource when it comes to planning your story. I use Pinterest daily, and have at least one board for each book idea I think of. It was very easy to organize one's thoughts this way.
1. Characters
Character inspiration doesn't always come easily. Make a board and save every interesting face you come across to it. Characters often develop on their own in your head, and it is very hard to stick with just the original idea of them, but pictures can help in description writing.
2. Cover Photos
I have a board that I save every perfect image for any book that I could possibly write. If you have a specific idea for a book, type in a key word and add "aesthetic" to it. This will often help my results. I add "aesthetic" to just about every search on Pinterest.
example: "bad boy aesthetic"
3. Setting
Can't think of how to describe your character's house? Or their town? Look it up. Type in as many keywords as you want and scroll until you find a perfect one. Or combine three.
example: "big white mansion", "log cabin lake"