I have an idea for a science fiction, younge adult novel, perhaps even a trilogy (if time, patience and willpower allow)

I've been thinking this idea over for about a year, now. I've gotten a few chapters into written form, but that was a while ago, and at this point I'm beginning to hate them. It was actually for a Class Assignment on creative writing. But the idea had been there for a longer amount of time.

Anyway, I hate what I've already written, and I 'm planning on just trashing it and starting over. The problem is. I don't know where to start. If I should start before the mutations (genetic manipulation, inhuman, etc) or after them when the main characters who have been mutated have been rejected by society for assosiating (and fighting for) with those who aren't mutated.

I also don't know HOW to start it. I want to grab the reader, drag them into the story by the collar of their shirt. But I can't figure out how.

Just so you know, (as if it matters) I'm 16, 17 in January. 
And I've tried writing three other novels, gotten about twenty chapters in and realized it wasn't what I wanted. Maybe I'm afraid of that happening again?  At least for this one I actually like the plot and it makes sense (to me at least)  and there aren't nearly as many plot holes...and it's not horribly cliched.

-_-' At this point I'm just whining. I'll stop now.
Any suggestions to kick-start this novel that's been brewin' in my head for a year?

I once read something that Diana Gabaldon(author of the highly rated Outlander series) wrote about how she writes. It really helped me.

She just writes. She writes a snippet here, a snippet there. A part of a chapter, a whole chapter.

She just writes it then later pieces everything together. She said she might have pieces of paper floating all over the place, but at least she's got down on paper what she wanted to.

I do this now. Mine is all in my computer, so I put little arrows and stars, with a "put this in this chapter", or "this should happen before this does" !!

It has really helped my creativity. I know it's not a system for everyone, but it worked for her!

Hope this helps!

Just write everything, quit worrying about how the book starts and just get the story out!

I recently heard Paul Flieschman doing a talk and he told us he had an idea in his head for 20years before it came out and he could write about it (Saturnalia was the book).  He said to just write what you need and for a 100 page book he has 600 pages that did not make it in.

Other thing is, backstory really helps if you are writing a triology, having everything worked out in the history of the world etc will really stop headaches down the road. rewrites always happen too, I am on my third soon to be forth rewrite for a fantasy idea I have. 200 pages of story I do not like.

Dont trash it. Save it. You may read it later and realize it wasnt that bad and just needed some fixing.

And for how do I get the story out. I just write I sit down and write and I dont worry about form or fleshing it out or anything but getting the basic story out of my head and onto the screen. Then when I finish it I go in and clean it up and make it pretty.