Write what you know
“If any man wish to write in a clear style, let him be first clear in his thoughts; and if any would write in a noble style, let him first possess a noble soul.” - Johann Wolfgang von Goethe
To be a successful writer your readers must trust you. It is easiest to gain your readers trust when you write about your area of expertise. Why? Because you actually know something about the topic your are writing about.
If you don’t have an expertise then start reading and writing about a topic, it is the best way to develop and show off your expertise.