Be not afraid of going slowly, be afraid only of standing still.
When I was a child, I was gifted with an extremely fervid imagination. I would make up stories and I would sketch them, write them, then get my Barbies to enact them. Really, I was a bit of a storyboard artist in the making. Then I grew up and the more formal my education became, the more limited my imagination felt. To this day, I struggle to let go of my preconceptions and my writing anxieties; although I have toyed with one storyline for almost four years, I haven't progressed past the first few paragraphs which I re-tell myself almost every night, before falling asleep. Try to begin in a place that challenges your acquired ideas of what you are most able to write about. Begin with:
It was a dark and funny night.