I have to prove how the narrator in "The Tell-Tale Heart" by Edgar Allen Poe is reliable for a debate.

Readers can consider the narrator of "The Tell-Tale Heart" reliable in that he was the only person present at the time of the murder besides his victim and would therefore know where the corpse was concealed. He opens the story by admitting that he has a disease that he calls a "nervous disorder," but he denies that he is mad. He seeks to prove his point by how objectively and calmly he tells his story.
The narrator must be reliable if we follow how methodically he plans the murder once he had decided he needs to commit it. He takes the reader through the nights leading up to it, how he wins the old man's trust, and how patiently he gains access to his bedchamber. The narrator's account of how he planned and executed the murder is detailed enough to be credible. He is intimately familiar with the old man's room and self-aware enough to know that he must hide all evidence of the crime.
Though the narrator confesses at the end of the story, it does not harm his credibility because he indicated at the beginning of the story that he is afflicted with a nervous condition. His nervousness at being caught gets the best of him and he mistakes the nervous beating of his own heart for that of his victim.