Problem

I think one of my biggest obstacles in writing (after lack of time, of course) is creativity. Not coming up with new, exciting plot twists, but new story ideas that capture my attention for a day- a week. They consume my creative juices to produce a workable idea, then get filed under "Ideas" in my writing folder. They're all great ideas, of course, but none of them will ever amount to anything if I don't learn to put them aside and dedicate my mental facilities to a single storyline for a while.
It sounds to me like I have a lot of problems and no solutions. Thus far anyway.

Plotting

"I'm plotting. Be scared"

Actually, you don't have to be. Because my plot has hit a road block.
When I started writing Leesha's story, I figured out the beginning, her goal, the end, and a scene somewhere in the middle. I have a rough idea of what happens in the middle, but I'm at the beginning. This is a problem. I don't know how to send Leesha on her crazy quest! Plus, I'm trying to edit what I have so far (a pathetic 20 pages or so) for the writer's conference in May...

What is the best way to plot out a story? I need to figure it out or my fledgling novel will never fly!