This page of the guide to Dragon Ball Z: Kakarot has a walkthrough for the Wild Imagination side quest.
Time frame: From Attack of the Saiyans - Episode 1: Bulma's Advice - Episode 1: Interplanetary Preps
Reward: 6,028 EXP, D Medal, Super Sacred Text of Martial Arts
Where to start: West Area, as Gohan.
Talk to Krillin and then fight two opponents. The first of them will be Nappa.
The second is Krillin. Defeat them to complete the mission.