I know I'm kind of late to the party, and this is an old post, but I think I can give you some advice, pal.
I've been screenwriting short films for years, and even a play once. Knowing that you'll be spending a LONG time doing something really boring is always a deterrent. Sometimes you sit down in front of a blank screen, and after a long time, you realize you're just not motivated.
Here's some tips. Bring a notepad everywhere you go. Whether you're driving to school, at home, or just out with friends, some inspiration or idea may just come to you, and you'll be screaming at the world if you don't have a pen and paper to put it down on.
Another thing to do is build a structure. Stories often begin with a lead-in to the protagonist(s), and then a shocking revelation, and then a twist, and finally a climax. Don't stick with this concept entirely, get a little loose. But keep it within those lines and you'll have something that will be hard for a reader to put down.
Also, work on the dialogue. Pick up a thesaurus and try some new, fascinating words. The most topical, untrodden, and enchanting dialogue will perpetually bewilder and astonish the curious bookworms.
It's not much, but dang... it's something. Good luck with your novel, and please hit me up when you near closer to it's publication.
EDIT: Forgot to throw this in; get into the mental habit of trying not to stop writing or typing a sentence at all. Even if you want to change/edit a word, you can go back and do it later. You can simply/easily add/apply a slash to make/create/spawn cool/interesting/awesome words and events/occassions without loosing your concentration/motivation. When you are the most fluent/uninterrupted, you become/are more poetic/philosophical/driven to keep writing/typing.