How do I do this?

Hmm... you seem flustered because you can't see the rice. Take another look around to see if there are any tillable tiles. You'll probably be able to harvest soon after cultivating the field.

Go out to your territory, build a farm or something, and then you can gather resources.

Ugh, so annoying.

On the bottom right. If you press the return button, you'll see the farm you built above the cathedral building. Touch that to harvest the rice you've grown. On the left. You can also enter the categories marked with red dots on the right to find ways to obtain them for free.