The Fighting Dojo is located in the north-western part of Goldenrod City, just south of the Goldenrod Radio Tower.
To find the Dojo Master, you need to:
1. Enter the Fighting Dojo.
2. Talk to all the trainers inside.
3. Defeat all the trainers in the Dojo.
4. Return to the Dojo Master who is standing in the back.
He will then challenge you to a battle. After defeating him, you will receive the TM for Low Kick.