Useful tip: planting a seed will automatically harvest whatever is planted there. You don't need to slash the plant with a sword and then plant a new one, you can all do it in one fell swoop.
At least it was like that a while ago, haven't bothered with the more labor intensive sorts of farms for a while.
Also, another newbie tip: Lava is great for moats and as a flaming barrier of doom on your rooftops. If you only put half a bucket in or make the pit wide enough the items dropped by the enemies will not be burned, so you can build a tunnel below the pit and collect all the drops.
Carrying Liquids around is a pain, so another tip: Build a sideways E sort of structure, open side up, only with the pits a bit deeper as a reservoir for the rare case where it does not work.
In-> |_I_i -> out
Drop a bucket of lava or water directly on the middle spike. It will drop down on both sides. You can scoop up one of the pools (in the above example, the left one) and repeat the process, creating infinite amounts of water or lava (which will flow out to the right). Very convenient if you are planning on flooding a structure high up in your castle, for example to farm piranhas or goldfish - or for the waterleaf. It's not fast, but beats running around like an idiot.
You can also build a cascade of dividers and stuff that runs infinitely without any user input, but that's hit or miss unless you know what you're doing. It also has the ability to flood the entire bloody map if you leave it unattended, so don't experiment with it on a map dear to you.
Please note: some servers frown upon this liquid generation stuff, since it is an exploit of the game engine and can in turn be exploited by trolls. Still, beats lugging a lake to your Obsidian generator one (inventory worth of) bucket at a time.