Instructions
Enter the underground area at the Game Corner in Goldenrod to get a uniform. This will allow you to pass by the guards at the nearby Radio Tower.
Fight your rival near the stairs of the tower after he recognizes you. After winning this fairly easy fight, head upstairs into the tower.
Defeat the rocket grunts who you encounter on the second floor. If you are low on health, you can bypass most of these fights. Head up the stairs to the third floor.
Beat more rocket grunts on the third floor, head up the left staircase to the fourth floor and then continue on to the fifth floor.
Save your game and then speak to Petrel, the Team Rocket executive. A fight will begin. Attack Petrel's Pokemon with Psychic-type moves such as Psybeam and Confusion to win the battle. Petrel will give you the basement hideout key after he admits defeat.
Go back to the underground area near the Game Corner. Head to the right and unlock the shutters with the key to open the path to the basement.