Gardening: A primer by a Grandmaster Gardener
First, visit the gardening NPC in Wizard City. As you level, there are more advanced NPC's in Krokotopia, Mooshu and Celestia, each selling better seeds and spells. Buy some basic seeds from the first NPC (dandelions, etc) and get started. Pick a clear area that fits into your land area of your house. I've never used the planters offered as I have a house for each of my active characters.