Walk with the arrows or W A S D. The white square is what you're pointing at.
SPACE uses whatever you're holding on that square.
1 axe 2 pickaxe 3 hoe 4 watering can
5–8 seeds 9 fence 0 wood path
Hoe the grass, plant a seed, water it, then go to bed. It grows overnight.
Your farm saves by itself. Press X to start a brand new one.