Writing what you know doesn't mean to limit your writing in a way to make everything too familiar or cliche. It means to use your life experiences as a jumping off point for your writing.
Writing what you know can help you develop details that bring a rich quality to your story structure. It can also bring about a familiarity that helps give you confidence to create unique characters, visualize setting, and infuse action and tension in crucial scenes.
Some of the most memorable characters in literature are based, even loosely, on someone the author knew. Almost like the author used their best strengths or limitations and amped them up or down.
When I begin writing a novel, I like to use names of people from my hometown for characters. I even use some of their idiosyncrasies to develop that character like accent, gestures, hidden talents, even voice. Then, I amplify those tiny distinctive qualities to make a totally new character full of rich detail and life.
What ways do you write what you know?