The first step to writing a memoir is to collect data-- memories, photographs, letters, journals. Gather and collect hard copies from anyone you know who might have such material in their possession. If they don't want to part with it, you can ask for copies. Old photos can now be scanned into the computer and copies made nearly as good as the original. The elusive and frustrating task of collecting memories that exist only in the mind of another can put anyone off, but there are ways to unlock that memory bank despite obstacles. I'll post more on that later.
Uncut Diamonds, my semi-autobiographical novel, came from my huge collection of personal journals. These contained memories, incidents, even the weather, which I weaved into the story, making stuff up as I went along. (Too much is fictionalized to call it a memoir.)
Farm Girl came from two basic sources. 1) The memories of my mother, sharp and clear, as she talked and I typed on my laptop. 2) The written stories and reflections of my grandmother who was a writer as well as an artist and photographer.
Collections already exist in some form, and it's up to the writer (who in folklore terminology is called the "collector") to find and gather. Sounds simple and obvious, I know, but sometime it is the obvious things that escape us. Writing will come later. First comes gather and collect. Go through your closets and files, and gather the data all in one place. Call up relatives and see who has what and is willing to part with it, or at least share. Older people especially are often eager to talk about the past, and to share their memories and photos.