"How to Write your Life Story," by Ralph Fletcher, presents an interesting and entertaining introduction to writing about the most important people in the world--yourself and your family. You can get additional ideas from larger books, but this little book, written for young people, covers the most important points about writing an autobiography.
It debunks a few common lies that people use to convince themselves they don't have a story worth telling--that you have to be a famous celebrity, have an amazing life, or be old and gray.
You already have everything you need to start writing: a character (yourself), a setting (places you have been), and a plot (the events that happened to you). Fletcher recommends making a list of memorable events and then writing about the ones that seem most interesting. These events will probably be the ones that are told and retold every time the family gets together.
He suggests gathering up the artifacts that have soaked up your personal memories and holding them in your hands until the memory rises to the top. Drawing maps of important places in your life, and a heart map of the things that really matter to you, will help you uncover the treasured and terrible tales you want to tell.
Whether you choose to write a group of vignettes, a chronology, or a multi-genre collection of short pieces, you can always switch to another form until it feels well-suited to the story you're trying to tell. The most important message in the book is that writing about your life will help you understand it better. So, read the book and start writing.