What does Katara say at the beginning of every episode?

Katara doesn't actually say anything at the beginning of every episode of Avatar: The Last Airbender. There is a common opening sequence that features different lines from various characters, but Katara's is not a consistent opener.

The most common opening line you'll hear is:

"Water. Earth. Fire. Air. Long ago, the four nations lived together in harmony. Then, everything changed when the Fire Nation attacked. Only the Avatar, master of all four elements, could stop them. But when the world needed him most, he vanished. A hundred years have passed, and the Fire Nation is closing in on the Earth Kingdom. The Avatar must be found."

This narration is then followed by scenes of the main characters and a glimpse of the conflict.

While Katara has memorable lines throughout the series, her dialogue isn't featured at the beginning of every episode.