Start where ever you want to start. What moves you most? Was there a defining moment? What made you want to record her story in the first place? If you're unsure, you can always just start at the beginning.
It has only been a few weeks and your emotions are still raw. You may not wish to publish what you write now but you will no doubt discover the seeds of ideas in what you write in this initial pass.
I have always wanted to write my grandmother's story but she has long passed on and no-one from her childhood is still living. If I write her story now, it will be a work of fiction.