Most people start with rock because it's generally the position your hand is in already when counting down, so you're better off picking paper.
But wait. Most people know this by now, so they'll probably pick scissors anticipating your selection of paper, so you're better off picking rock.
But wait. If they thought of that, they'll pick paper. Plus, if they didn't know that most people know that most people start with rock, they will also therefore pick paper, so you're better off starting with scissors.
But wait....
Has anyone got a piece of paper and a pen I can borrow?