A seed. These come in various colors and varieties, and can be found on boglings and player vendors (use [sv seed).
A water pitcher can be purchased at any inn. A water trough or water barrel comes in handy to refill.
An empty planting bowl can be purchased from the provisioner (2gps each)
fertile dirt (regular dirt can be used).
greater cure potion
greater heal potion
greater poison potion
greater strength potion
potion kegs. These can be placed in your pack to make it easy to care for many plants.
Next Step planting your first seed: Planting Basics
Bring your purchased bowl to a patch of dirt. Double click the empty bowl.
target the dirt to fill the bowl. Dirt can be located at any farmland or dirt path, or bought from Vendor stone at Brit bank. 40 fertile dirt can be used to fill the bowl.
Now you have a bowl of hard dirt.
Double click the pitcher and water the bowl twice to turn it into soft dirt ready for planting. NOTE: If you use too much water it will say "A bowl of squishy dirt". This will drown your new seed. If this happens, double click the bowl, find the empty bowl on the bottom right of the menu, empty it and start again.
Double click your seed and target the bowl of soft dirt to plant it.
That's it! You now have a new plant started. Follow the links for care instructions.