After accepting the mission from Professor Hastings, head west out of Ringtown. You'll quickly come across the broken bridge. Speak to the construction worker there, who will explain that a large boulder is blocking the path, preventing repairs. He'll also mention that a Pokémon with a 'Crush' Field Move might be able to help. This is your cue to start looking for such a Pokémon.
Explore the area around the bridge. You'll encounter various Pokémon like Bidoof and Geodude. Your goal is to find a Pokémon with the 'Crush' Field Move. Geodude is a prime candidate for this. Capture a Geodude by circling it multiple times. Once captured, its Field Move will be available.