Writing solely about what you know is misleading advice. To write is not to relive your own experiences, but to create new worlds and outcomes you have imagined. In the real world things happen without rhyme nor reason, but in the fictional world you create, you are the director of how the story unfolds. Fiction, unlike life, needs to be logical and build meaning for the reader, while real life can be chaotic and disjointed. Even if your novel is a science fiction, in a far future world, your characters need to be identifiably human or explored using your own humanistic instincts.
Searching for that unique idea, that "never been done before" idea will lead to frustration. Every idea has been done before in some way or another. But what sets your idea apart is that it is written by you, with your voice and your distinctive vision.
Below are some tips on how to to keep track of ideas that present themselves to you:
—KEEP A NOTEBOOK. Ideas like to reveal themselves at unusual times. Get into the habit of jotting down ideas and concepts as they come to you in a journal. This should be small and with you at all times. Smartphone note-taking apps are suitable. The most popular times for ideas to hit you are during the shower or just as you dose off to sleep. So try to remember to write those ideas down as soon as you get out of the shower, and reach for your smartphone or notebook before you hit the wonderland of sleep.
—TEST YOUR IDEAS. When you come across a new story idea, or something you've been pondering, put it to an ideas test by seeing how the idea begins to bring up other elements of the story (character, conflict, motivation, plot, setting, etc.), asking yourself does the idea or concept lead to these elements, building step by step? If it does not, can you identify where the story breaks down? Does your character have a clear motivation? Does the motivation suggest a potential conflict in the story?
—EXERCISE THE RIGHT WAY. Practice building your story from an initial idea, by questioning character motivation, plot, and tone, or anything that comes up. Refrain from censoring or limiting yourself. Write anything that comes to mind about the story and keep finding new ideas and directions you can take with the premise.