Before you begin the next mission, check the crate near the house and take some new weapons. Check the medicine cabinet to heal your health.
At the side of the house is a crate full of ammunition. Refill your ammo. Make sure to do this before you begin any mission. It will save you a lot of restarts.
Once you've finished preparing for the mission, approach the van at the side of the camp and press E to start the mission.
After the mission briefing, go in front of the van and you'll find a bike. Use it and start driving towards the mission marker.
A bit of information about driving in this game, it's difficult. If you press the directional buttons for left and right for too long, you're going to swerve and go around. Kind of like drifting. Be mindful of that when you're driving.
I would also recommend that you change the bike for a car before heading into the prison. You're gonna need something bigger than a bike to mow the gate down. Once you're inside, leave the car and enter the prison entrance.
Shoot down the guard that's standing on the balcony, then turn left and go down the first set of stairs that you find.
Keep walking forward once you get to the bottom ground floor and head towards the small room below the balcony that you came from. A cutscene will play afterwards.
Now that you've freed the leader of the resistance, escort him back out the prison the same way you came and get yourself in a car.
Head for the mission marker to complete this mission. If in case you finished the mission at night time, then make sure to use a car instead of a bike. Bikes don't have headlights in this game for some reason and it can get really dark at night.