STORY OF SEASONS: Pioneers of Olive Town

Tilling will come from using the Hoe (provided by Victor during the first few days of the tutorial) on farming land. This is required to plant crops (but not trees), but you can easily dismiss tilled soil with a hammer. If you want to get this trophy early on, just do that. However, you will be tilling a lot of ground for later chances of rarer crops or higher levels, so just wait for it to come naturally. As with most cumulative trophies, if you want to check the progress:

1) On the Title Screen, open the trophy menu and you can see the cumulative progress on trophies such as this.
2) In-game, open up your menu and go to the Records screen. By clicking on Change Title and opening Fieldwork, you can see how many times you've tilled the soil successfully.