Since your Bike is busted, you have no choice but to ride with Boozer. Hop at the back of his bike until you reach a blockade on the road. Help him push the Humvee off the road.
As soon as you push the vehicle out of the way, both of you will get attacked by Marauders. This will be your Tutorial to Melee Combat. Follow the onscreen instructions to fight back and kill the attackers. Once the battle is over, hop on again on Boozer’s Bike to continue with the mission. Mission ends as soon as you reach the tunnel. The next mission will immediately start after.
We have questions and answers related to this topic which may also help you: