I have always loved writing started as mainly fan fiction then developed into using The Sims to create my characters and write the story's. I have been writing with an idea I used to role play as a child.
But as soon as I sit down to write, BAM block! I have always been critical if myself (thanks mom) but I can't seem to write it down? I can talk as if I'm the any of the characters, and find myself running scenes while doing dishes.
I agree with above - you just need to write something, anything. Write something every day until it becomes a habit. Someone (I forget who) said they make sure they write so fast that their inner critic can't keep up.
Accept that your first draft will be crap. If it will help to keep the momentum going, don't stop writing to go back and change details. If you want to make a major change to a character, story arc, whatever, just note it down on a sheet of paper that during the re-write you need to change detail x up to page y to fit in with new detail z. Then carry on writing as though the change has already been made.
Also, read War Of Art and learn how to just get on and 'Do The Work' like a pro!