As anybody who's read Little Women or Anne of Green Gables could tell you, you need to write what you know. Problem is, unless you're writing non-fiction, you still have to make some stuff up. Most likely, this will require learning something new. In other words, you can still write what you know, but when you're introducing characters whose lives are not exactly like yours, you have to expand what you know through research.
I'm almost done my second novel. I think there's just one more chapter to write. Of course, I've said that before. I've been learning stuff as I go, but I will probably want to go back and add details when I revise.
Here are some things I've researched before or will research more in the future for my Rocky book: the Rocky movies (do I really have to watch them? Those of you who knew me on my mission will appreciate the irony), boxing, The Merchant of Venice, foster care and adoption law, Dodge Ram trucks, shack building, karate, separation and divorce law including custody issues, Kingston Penitentiary's rules and typical inmates, Ontario high school curriculum, and more I can't think of at the moment.
For my third book, I'm planning to do the research and plot outline before I get started. Here are some things I need to learn more about: Alzheimer's disease, Down syndrome, Utah politics and election campaigns, nursing homes (I used to work in one but it's been a while), soccer, Wordsworth, teenage pregnancy in the late 70's, institutions for the disabled. I'm sure there will be others as I go along.
Also, while I'm thinking about writing, I'm pondering titles. Sometimes they come easily, other times not. I already have one in mind for Book 3: A Forgetting and a Sleep. Book 2 has been much harder--I'm almost done and still don't know what to call it. My working title was Rocky Ground, but I've always hated it and couldn't think of anything better. I did have another idea, though. What about Dear Mr. Stallone? I'd love to hear what you think of my titles.