How do we use "one thing leads to another" in sentences. Is it for positive or negative things?

Neither negative not positive. It is just an expression that suggests that a number of unspecified events or actions took place, without going into details. For example, about 100 years ago, someday shot in Serajevo Archduke Ferdinand.

One country complained to another country, the other country threatened military actions, other countries intervened in the controversy, ONE THING LED TO ANOTHER, and World War One started...