You receive the HM for Surf as a reward for finding the Secret House in the Safari Zone. It is located in northwest section of Area 3, and you must reach it in less than 500 steps. You need to earn the Soul Badge in order to teach Surf to a Pok?mon.
To reach the Secret House, go east from the entrance into Area 1. Continue east and walk up the plateau, then back down the steps on its left side. Go over the plateau to the north, then circle around west to reach the entrance to Area 2. Walk until you reach the barrier, then go up and over the plateau. Walk north until you reach a line of bushes flanked by two trees, then walk east. Walk up and around the tree and west until you reach the tree line, then go straight south to enter Area 3. The Secret House is to the west.
Outside of battle, Surf allows you to cross bodies of water, allowing access to the Seafoam Islands and Cinnabar Island. In battle, Surf is a water-type move that deals damage and is especially effective against fire-, rock- and ground-type Pok?mon. When used in a double battle in "Pok?mon FireRed," Surf damages both enemy Pok?mon.