Check out this great speech on creativity by Elizabeth Gilbert: http://www.ted.com/talks/view/id/453.
The moral of the story: just get out there and make something. Be creative. Don't worry about failing.
I often find myself staring at a blank piece of paper unable to get the ideas out of my head. The hardest part is just getting started. As if I'm going to get it perfect the first try -- that doesn't happen. You have to write and idea down. walk away from it and come back when you're ready to refine it.