Requirements: Reach level 40+ and complete the Buu Saga.
After meeting these requirements, when you enter the city check all 3 main areas for mentors (There's only ever a maximum of two) and speak to Gohan and Videl if you see them (they'll be together).
Log out and back in to speed up the process.
Speak to them for a test where you'll win Eagle Kick.
Continue with the game or Parallel Quests and speak to them after each for more tests.
Second: Ki Blast Cannon.
Third: Hawk Charge
Final: Justice Combination and your trophy.
You are now free to change mentors.