This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Yamcha's Alive side quest.
Time frame: From Evil Emperor Frieza - Episode 1: Bulma's Advice - Episode 1: Interplanetary Preps
Reward: 6,028 EXP, D Medal, Awesome Book for Grown-Ups, Fancy Earrings.
Where to start: Western area, as Gohan.
You start the quest by talking to Oolong at the Capsule Corp.
Your task is to follow the specific person and collect information about him.
When you find out the truth, go back to the quest giver to get your reward.