Why/How does Eren remember how to activate his Titan power? Eren's father injects him with the Titan serum years before he first uses it.  So how is it that he remembers how to activate it?
I don't know how far the anime is yet, so I'll put it into a spoiler.


   The shift between human is triggered by any injury that causes blood loss. The first shift is triggered when Eren gets eaten by one of the titans and hurts his arm. To trigger the next one, he bites himself to protect Mikasa and Armin. Apparently, also the will to accomplish something is needed to become a titan (kill the titans, save his friends)