Influencing Parliament and Central Government

Parliament is the ultimate arbiter of how change does or does not happen. But it can seem arcane and baffling to those inexperienced in how it works. This can make it seem daunting to those who want to change or amend the law. Similarly with Whitehall and 10 Downing Street, this is where the power in our political system lies, but where to start in trying to influence these apparently impenetrable institutions?