You just start somewhere! Just pick a year and start there. Don't worry about things being in order (if you can do that)...My books are basically in order, but I often find pictures after I have done a book and add them in somewhere else. I try to put a date on every page (month and year, or just year if I can't remember).
We have people that join late in their families (scrapbooking), and they don't really have a goal. They just do what pages they can when they can.
When you pull out the pictures, sit and go back in time. The memories will come back to you and that is what you need to journal (write down the stories). My pictures often tell one story and then while I am scrapping, other stories come to me that were not captured in pictures. Those stories still get written down. (Maybe we were sledding and the kids said some cute things...I write that down, but then maybe we all went inside and made hot chocolate together...I write that down, too).
You family will appreciate ANYTHING that you can get down in a scrapbook. My pages are not anything that would be impressive to anyone else. Usually they are just the pictures cropped and a strip and a triangle or just a simple piece of paper under the whole page that complements the pictures.
HTH!
Jenn