In 2005 Series 1, when do we find out that The Doctor himself destroyed Gallifrey, the Time Lords, and the Daleks?
The first indication that we get that The Doctor was personally responsible for the end of the Time War and the death of both races comes pretty early, in S01E06, "Dalek", when The Doctor taunts the newly-discovered broken Dalek:
DALEK: I demand orders!
DOCTOR: They're never going to come! Your race is dead! You all burnt, all of you. Ten million ships on fire. The entire Dalek race wiped out in one second.
DALEK: You lie!
DOCTOR: I watched it happen. I made it happen.
DALEK: You destroyed us?
DOCTOR: I had no choice.
DALEK: And what of the Time Lords?
DOCTOR: Dead. They burnt with you. The end of the last great Time War. Everyone lost.
This makes it clear that The Doctor was personally responsible for the death of the Dalek race, and his statement that the Time Lords "burned with you" would also place responsibility for their deaths on him as well. I don't know that he ever explicitly mentions destroying the planet Gallifrey itself until much later, possibly not even until he meets The Master, I'll have to watch those episodes again to be sure.