|>>|| No. 6948
I'm no expert but there are two basic routes. Some people work on developing in-depth characters then put them into a situation and sort of write along to see where it goes. The other route is to start with a basic plot idea then start figuring out how that would work, breaking the premise down into component parts; scenes, setting and characters needed to pull it off. It has to follow a basic structure with character motivations and things fleshed out for at least the principle characters to seem real and distinct, with believable behaviours.
Once you've figured that out, you arrange the scenes into order in chapters and the chapters in order of the book itself then keep adding detail to each part, rearranging as you see fit, until you feel confident about putting it into prose. At which point you do that. Then you basically do the whole thing again over and over as you realise what you've done doesn't make sense or is missing a key character arc or that you've made a really stupid amateur writer's error and as it's not a YA novel remarkably similar to any of the most recent best selling novels the year you finish writing it (as opposed to started), nobody wants to publish it because they don't expect to bank on it.
Everyone does a weird mix of the two routes though, nothing's ever "purely" one or the other. You just figure out what you're doing as you go along.
It's best to write it second person, present tense or you'll be setting yourself at a disadvantage from the start, if you ever want it published. Oh, and remember to back it up constantly or you'll definitely lose enough of it at some point that getting back to where you were before it crashed seems insurmountable.
I can recommend pirating Scrivener to use as a word processor; it's pleasant to write into and has a file manager thing that makes it extremely easy to navigate and overview the work as a whole. It has all sorts of other bells and whistles you won't need but the first half hour in its tutorial thing should cover everything you will use.