Which ship was the first to use the newly adopted SOS distress signal?

The 'SOS' signal was established as an International Distress Signal by an agreement made between the British Marconi Society and the German Telefunk organisation at the Berlin Radio Conference, 3 October 1906. The signal was formally introduced on 1 July 1908.

The first time the newly adapted 'SOS' signal was used in an emergency was on 10 June 1909, when the Cunard liner RMS Slavonia was wrecked off the Azores. Two steamers received her signals and went to the rescue. The RMS Slavonia was a 10,606 passenger ship that was built in 1902 and named the Yamuna for the British India Line. She was later sold to the Cunard Line in 1903 and renamed Slavonia.

This was nearly three years before the Titanic made her famous signal!