One of the first choices to make in writing a memoir is deciding on the focus of your memoir. Through which lens do you want to tell us about your life? Is it a particular season, time period or theme?
Once you have chosen your focus for writing your memoir that is the lens you keep to in telling us your story. You don’t need to give all the background to your life, where or when you were born, who all your relations are, and where you fit into your family. A memoir isn't a family tree.
It's not that any of that information should be excluded, but with memoir writing, it is included only in as far as it serves the focus of your memoir. As William Zinsser has said, memoir assumes that your life extends beyond what you share. It's important that you stay true to the chosen focus of your memoir.
An autobiography, on the other hand, is the whole shebang.
While a memoir focuses on an aspect or particular time of your life, an autobiography spans your entire life. It details your life from your birth, through kindergarten, to the name of your most influential teacher in Grade 12. It extends to the details of all your loves, your first job, the highs, the lows, and all the in-between bits of your life. It is typically a chronological account.
Having said that, there are a few considerations to keep in mind which will help in determining the specificity and the detail of your autobiography.
Who are you writing for now? Which readers in the future will be interested to know about your life, and why? Are the more important elements of your life related to the era in which you lived, the circumstances you lived through (the Great Depression, World War II), the talents and knowledge you have (the evolution of opera over the last four decades), or is it who you are (musician, mayor, or mischief maker)?
Think history textbook when you think autobiography. Think short stories when you think memoir.
Which is it for you? Memoir or autobiography?