Eren's first transformation into a Titan occurs in Chapter 13 of the manga and Episode 9 of the anime, titled "The World the Girl Saw: The Struggle for Trost, Part 9." This pivotal moment takes place during the Battle of Trost, a crucial conflict between humanity and the Titans.
"I'll kill you... I'll kill every last one of you! I'll drive you out of here... I'm human! I'm human! I'm human!" when does eren first transform