At the rebel base on Yavin IV, the rebels analyze information in R2. The Death Star enters the Yavin system.
During the attack briefing, the rebel commander explains that the Death Star's defenses are designed against large-scale attacks, but a small one-man fighter could penetrate those defenses.
There's a small exhaust port that leads to the reactor core.
One torpedo in that thing would start a chain reaction. Then it's bye-bye Death Star.