I have lots of seeds and saplings, how can I plant them? I have Dry Dirt, Dire Stone And Dry Sand (and no equipment except a Watering Pot).
Get your Hoe to do it!
Hoes will prepare the soil for planting. Once your dirt has been tilled, you can plant seeds. You'll see the top of the block changed to the appearance of "fresh tilled dirt".
After tilling the dirt you can also you use a Watering Can to improve the speed at which the plant will grow.
Further along in the game, you'll want to automate the watering with a Sprinkler.