A lot of aspiring writers, myself included at some point, hold the belief that there is a formula, a fixed set of guidelines that can be adhered to determine how you begin to write. The sad truth however is that there does not exist anywhere in the world such a formula or such guidelines.
Yes there are books you can read, and workshops or classes that you can attend, but none of these will give you the answer, ‘Where do I begin?’ At best, and this is why they are important, these books and guides will give you tips that you will find useful once you have begun, which is to say the answering of that question ultimately is left to you to answer.
Every person who wants to write has a story to tell, which only they know how to tell, in other words every one, depending on what they are trying to say will go through a process that includes trail and error, false starts, a process of writing, and writing and writing and writing again, until at the end of this process they begin to feel their story take shape.
If were to forced to answer this question in a direct way, my answer would be ‘It does not matter where it is that you begin the important thing is that you actually begin, and stay with the process until you feel like you have said in your story what it is you wanted to say.