Starting can be hard. It very often takes me a long, long time to edge up sideways to starting an idea. I have to "nerve up" in order to start a book or short story. And usually that means lots and lots of thinking.
Sometimes it's just finding the right situation for the opening scene. Sometimes it's down to an opening line. Most of the time though I just have to think about it long enough to make starting possible.
It ends up taking as long as it takes. But once I start the dam breaks and I can get going. But getting that first word on paper can be nerve wracking.
How do you start? With a scene? With an idea? Do you do a lot of pre-writing?