For someone getting back on a bike after years away, or riding regularly for the first time as an adult, the biggest mistake is over-buying on features before knowing what kind of riding will actually happen.
Start with how and where you'll ride most: flat paved streets point toward a hybrid or roadster, mixed terrain toward a hybrid with a bit more tyre width, genuinely rough or unpaved ground toward a mountain bike. Riding style you'll do 90% of the time should decide the category — not the 10% occasional trail you might try someday.
Beyond category, prioritise a correct frame size and a comfortable saddle height over gear count or suspension — a well-fitted simple bike is more pleasant to ride than a feature-heavy one that doesn't fit. A five-minute in-person fit check catches most of what matters before you've committed to a purchase.