This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Yamcha: the Man, the Myth, the Legend side quest.
Time frame: From Evil Emperor Frieza - Episode 8: Intermission to Episode 8: Intermission
Reward: 38,616 EXP, D medal, Tofu, Wolf Meat
Where to start: West Area, as Gohan.
Start by talking to Puar at the Capsule Corp.
The only thing you need to do in this mission is to fight Yamcha. This isn't a difficult task. The enemy's level is much lower than yours.