Brainstorming. I bought a notebook and pens. Literally. I'm sure I'll say it a million times, but I'm pretty technologically challenged. I make tons of lists--always on paper. I'm old school and proud! But once I had my notebook and pens, I started writing down ideas. I had a general idea of what I wanted to write about. Science fiction. Space ships, but not aliens. Assassins. A little romance. A strong heroine. And I just wrote down every random idea I had for probably twenty pages. I came up with potential character names, plot events, lines that I wanted to incorporate, character backgrounds, etc.
I recently reread my brainstorming notes, and maybe ten percent of what I wrote down actually made it into the book. I definitely wouldn't skip this step though. The process of thinking through random storylines and the creativity from letting my mind wander was invaluable. It got my mind working and helped narrow down what I liked and didn't like.
I think the most important thing to remember at this step is not to sensor yourself. No one will see these notes but you, unless you show them to someone. If you think it, write it down. Even if you think it's stupid or silly later, maybe it was an idea you built upon somehow. So don't edit yourself. Write down everything. Don't think about it. You can start to think when you outline. :)