There are established conventions within each genre, (such as crime, sci-fi, romance etc.) and it’s well worth researching these genres to see what’s expected. Once you know the rules, you can decide whether to follow them, or subvert them. In choosing a specific genre or style it’s also worth considering how you think – do you prefer to follow rules, or experiment? Do you plan everything in advance, or are you spontaneous? Playing to your strengths instead of trying to make yourself ‘fit’ will help your writing be authentic. It might be that you prefer to let the character(s) guide your writing rather than choosing a specific genre. You could do some exercises to help you know them deeply – writing a biography which includes their personal possessions, problems and secrets, or experimenting with the ‘voice’ of your main character(s) and the rhythms of their language. Allow time for this experimentation – no writing is wasted. Once the character’s voice is right, you can let them tell their story, in their own way.