Answer:
Figure out what genre you want to write and develop some characters. Make sure that your main characters are decently dynamic or the book will be boring. as an aspiring author myself, I've actually been developing and changing my plotline and characters for like a year but you don't have to. find a nice, quiet place to start writing, focus, turn on some music, and jot down some quick intro ideas and lines that sound interesting to you. After that start your rough draft and remember, don't edit as you go, and write as much as you can till you hit a point you know what you're going to do next so you wont be stuck next time. :)