Hello! Honestly it depends what you want to go for. For things like fan fiction I find it easy to make a sort of tree that is basically a timeline of what you want a general plot to be, and then from that branch out aspects that you may want to add like character background, character development, a side story, new character, ect. Ofc researching things that you might want to add to a story but are not familiar with is great too in order to make things more realistic. If you’re creating your own story with your own characters you’d want to go through the exact same process and also do it with any characters that you want to create. So basically creating a life story of your character is simple bullet points that you can refer back to if needed for things like personality traits or family and friends. If you want to know anything specific about writing in general feel free to reach out!