how do i start a college essay??

Is this a specific prompt or the common app? In general, I would suggest you first brainstorm to find either a personal experience or a character trait that you think encapsulates you in a positive way. A good story never hurts, but if you think yourself excessively diligent or something like that, then focusing on a character trait can be effective. Once you have the subject matter chosen, then the trick becomes how to frame it. However, to start, I'd say the main challenge is figuring out which topic can, in one page, really illustrate what is unique and valuable about you.