Instructions
Fly to Goldenrod City and walk to the radio tower. You will be turned away by the Team Rocket members guarding the door. Travel south to the underground section of the city and talk to the Team Rocket grunt near the camera equipment to receive a Team Rocket uniform, which you wear automatically.
Walk back to the radio station. The guards will be fooled into letting you in, but your rival will show up and spoil the disguise, causing you to have to fight the Team Rocket grunts.
Defeat the Team Rocket grunts using your best Pokemon and proceed through the radio tower. As you continue on the path, you will have to fight multiple Team Rocket grunts until you reach the fourth floor of the radio tower.
Talk to the radio tower director to expose him as Petrel, a member of Team Rocket, which triggers another Pokemon battle. Defeat Petrel's Pokemon to receive the key to the underground. Petrel uses several level 30 Koffings and a level 32 Weezing, so plan your Pokemon team accordingly with Pokemon that are strong against Poison Types.
Walk to the south entrance of the Goldenrod City underground and use the key to enter. You will have to face your rival again, so plan ahead by putting your strongest Pokemon in your party. After defeating your rival, continue through the underground.
Activate the right gates to move forward and reach the second level of the underground. You will have to face multiple Team Rocket grunts during this endeavor. At the end of the path you will find the real director of the radio tower, who will give you the card key for the radio tower. This opens every locked room in the radio tower.
Return to the radio tower and use the card key to gain access to rooms that were previously inaccessible. Open the locked room on the third floor to confront the Team Rocket executive, Proton, and continue the game's storyline.