You should write about something that excites you. Think about the kinds of books you love to read, and make a list of what you love about them. Dystopian sci-fi set on far-off planets? Real-life stories starring characters with an attitude? If you enjoy reading about it, you’ll probably enjoy writing about it too. And as for not knowing how to write a novel, that’s how every author starts! The best way to learn how to write is to write, and if you need more help, check out these two resource pages for workbooks, pep talks, videos, and more:
https://ywp.nanowrimo.org/pages/learning-resources
https://ywp.nanowrimo.org/pages/writing-hub