You have to start somewhere, but you have to concede that the very first quest of Dragon Ball Z: Kakarot is more of a tutorial than anything else. Just to start your adventure with the sayans, what better way to start than to go looking for old underwear magazines for that perverted old Master Roshi? More seriously, apart from the childish simplicity of the latter, don't hesitate to review the Training Board, the Soul Emblems and everything that could be useful for your progress.
Master Roshi's Prized Possession
Master Roshi accuses Turtle of stoling his Girly Book. After finishing the dialogue, go chat with Bulma, who will teach you the position of Turtle: she is hiding behind the house. Go ahead and ask Turtle where the magazine is hidden, she will point to a pile of dirt between two trees. Pick up the magazine, bring it to Master Roshi and it's all. Told you, easy as pie.