In order to do a mission just go to the Hero Hub or Villain Hub location where you can request a mission. Depending on the way you handle the mission you will receive an amount of stars, each mission will give you at least one star.
Training Pack (Level 1-5)Edit
If you're a level one or five user then the missions that you will request will mainly focus in training exercises like shooting arrows, lifting weight and such.
Beginner Hero Pack (Level 3+)Edit
Starting level three missions will be more frequently in the city and will focus less on training. You will receive missions like rescue a civilian from a burning building, stopping robbers from robbing a bank, etc.