Littlebee,
I'm going to step in with the hope of, if nothing else, saving you from the pummeling I sense you are in for.
It's clear you're green; that's okay, we all were at one stage during our love affair of writing. My advice? Put your huge aspirations on the back burner. I speak for myself, and doubt few will admit what I'm about to, but I, too, was once filled with delusions of grandeur. Thinking I would tear the literary world to shreds and no one could touch the likes of me.
Well, the truth is, I sucked. Not only did I suck, my words were about as neatly strung together as vomit splattered all over the toilet bowl.
You're excited about this idea; that's great. You seem to have a passion, too; again, great. My only wish for you is that you are careful: don't let your emotions guide you through the writing of this book. The results if you do? A story with an odor reminiscent of dead rodents soaked then boiled in vinegar.
Write hard. Write honest. Write well. Find a critique partner, someone with no emotional attachment to you or your work. Get knocked back a peg or two, then fix what is wrong (and I guarantee you, there's always plenty that is).
So yes, to answer your questions, FIRST AND FOREMOST write the hell out of your story. Then revise, revise, revise. After that, revise some more. Work your fingers to the bone, until your novel is the best it can be. After all this, YES, find an agent. Clearly you have reveries of success; an agent is the most likely person to help you attain them.
You have many peaks and valleys to traverse, and it's a helluva ride. You bring back a lot of memories for me.
And I wish you the best of luck.
Lyx