To take the bull by the horns means to deal decisively with a difficult or dangerous situation.
This expression probably originated in the American West where it was a common, but dangerous, practice to wrestle (wrestle: struggle with a difficulty or problem.) with bulls. This was not only done for entertainment at rodeos but was part of the everyday working life of ranchers and cowhands throughout the west.
e.g. “she decided to take the bull by the horns and organize things for herself”
e.g. “Why don’t you take the bull by the horns and tell him to leave?”
e.g. “I took the bull by the horns and confronted him about his drinking.”