Try writing about something that you enjoy... 1. talking about 2. thinking about Or! 3. interested in You can start there and gradually plan forward. Try writing about something new and fresh that has well developed characters (depending on genre you plan on writing about). Good Luck! ^-^
There are many good writing processes. JK Rowling, author of Harry Potter maps out her entire books beginning to end before even starting to THINK about writing the actual book. Some authors take a random sentence like "the kids sat in a circle" and just write it out from there ( if they need inspiration). Or if you want, try writing just one chapter. Tell yourself what exactly you want in the chapter, then write it. Continue on like this until you have the entire book written. Also, think of character names first. If you need any kind of knowledge of science of math or something to make an element of the book realistic, research it FIRST.
The number one thing (at least in my opinion, anyway) is to understand that you're most likely not going to write it from start to finish. If you have ideas, put them on paper while they are fresh! You can always come up with away to begin telling your story later. I've had ideas for a simple conversation in my head that I thought was great, just a random idea pulled from my real life or something & I jot it down and kinda start building from there. Who's having the conversation? What's their relationship? What led to the exchange? What's their history, etc. don't be rigid in thinking that a story has to be built from beginning to end. I've started with writing something that ended up smack in the middle of my story. The start will come to you, just start the process. You'll figure it out. Keep a notepad or small recording device (my phone has one, handy!) & record ideas when they are fresh, you might not remember that great idea you had when you sit down to write it an hour or two later :-) good luck