You will see the side quest givers as blue question marks on the map.
The first one you can get is to collect four sugar bags in the Candy Kingdom.
The screenshot below shows the location of the quest giver, plus the blue and black diamonds show the four bags of sugar. You might have to fight some enemies as I believe there are 4 fairies near the grate and pillar area.
Warning: The first time I did the quest I used Finn to pick up all the bags of sugar with Y and then drop them off one at a time.
The second time I did the quest I tried to use Finn and Marcie to pick up the bags of sugar so I could do two at a time but it glitched and now I don't have any option to pick up the last bag.
Edit: seemed I cropped my picture too close. The top blue diamond should say "By the grate"