If you are playing version 0.0.5, after unlocking 3 letters at Vanya and talking to the Elf, after dialog the hero talks to himself as if he should talk to Saya, and the story should continue from there.
I know it's not a perfect solution because dialogues can be skipped with a single button, so you can miss something. I haven't refined it that much yet because the game is not supposed to be that linear.
A quest journal will probably appear in the future.