If you were to take a large enough sample of writers, you would probably discover that every possible starting point has been tried, and worked, for someone. Every storyteller is inspired in an different way from every other storyteller, and differently for each story. Every story is discovered via a new path that has never been traveled before, and will never be traveled again.
When starting a new story, game, etc., hopefully you have some starting point - an idea, a theme, a character, etc. Take your starting point, and ask yourself what that starting point requires. A character who is wizard is going to need a magical system in which he practices and a history - parents, birthplace, etc. A theme implies an appropriate conflict, which in turn requires a set of combatants and a context.
As you start to fill in details, it will be like tracing the components of a tree. If you start with a leaf, you will trace it to its branch, which will trace back to a larger branch, and so on until you eventually find all of the other branches, the trunk, and the roots. If you start at a root, it will lead you to a larger root, and then a large one, until eventually you find all the roots, the trunk, then the branches and the leaves.
My ideas are usually something along the lines of "What would happen if there was a person X living a world Y?" Where the properties of character X and world Y are defined only in the very broadest terms. From there, I can usually figure out how the story is going to end. Everything else must follow logically from those starting conditions to make the story interesting, compelling, and inevitable.