After leaving Cartman’s house after finishing the quest “Birth of a Coon Friend”, activate the first fast-travel point outside.
You have to defeat the sixth graders blocking the path to the next street to the north first in “Rise of the Fourthies” to reach the next fast-travel point. Simply activate the second fast-travel point in Main Street near Tom’s Rhinoplasty to complete the mission and unlock Fastpass as a Combat Buddy. There are more Fast-Travel Points in other locations in town. When you find one, simply “discover” it by standing nearby and pressing the spacebar.