To unlock the "Green Thumb" trophy in Yonder: Cloud Catcher Chronicles, you will need to find a seed and a patch of dirt on which you can grow a tree.
There are three ways to get seeds:
1) Buy one from a store vendor;
2) Random drop of a "mystery" (i.e.) random Seed from long grass (requires the Sickle, which you can purchase from Fairmont or earn for completing "Grass Needs Cutting" from Sly Mc'Wiley, south of the first village)
3) Random (but fairly common) drop of the tree's seed when chopping down a tree AND the stump
So the easiest way to get this early on is to cut down a tree with your Axe (speak to Minnie in Fairmont if you don't have one yet), chop up the stump too, and then immediately plant the seed that appears in the space where you felled the tree. If a seed doesn't appear just keep chopping away at other trees nearby (or Sickle some long grass) until one appears, go back and pop it in the dirt.
As a general rule you want to plant as many trees as you can (and replace the ones you chop down) throughout the game to keep the biomes happy, so this trophy will very likely pop up during regular play.