The majority of what I write evolves from a question, usually some form of a 'what if'. As a pantser, I often sit down to the keyboard with just that and let the words flow. The only constraint I set for myself from the beginning is whatever length I'm going for, flash, short story, long short story, or novel, because the approach is different for all of those.
I don't like knowing what is going to happen before I write it. It takes the excitement out of the process and that means I'm not at all motivated to write. I prefer to write like I read, manically plowing forward to find out what happens next.
For organization on novels, I keep either a notebook or a document open with all my character specs and world notes as they pop up during writing so I can attempt to keep them consistant throughout. Once I get to the second or third draft, I finally (for you planners having fits right now) get around to making an outline to check flow and pacing.
My advice to new writers boils down to: Sit down and write. Stories do not write themselves. As awesome as that would be, it doesn't happen. Finish what you write. That's the important one.
And when the words aren't coming, go do something else. Nothing sucks your motivation more than staring at a blinking cursor. I do a good deal of my writing away from my computer, running scenes in my head and then scrambling to the keyboard for five minutes to write it all down, then going back to what I was doing and working through the next few paragraphs.