After you have a simple farm up and running, you might want to consider adding good fertilizer and a backup water supply. Or perhaps you'll want these before you even plant your first crop.
The Compost Bin turns a combination of droppings and thatch into a decent quality fertilizer. You need Thatch in stacks of 50 to make the Compost Bin work. Add at least 100 or 200 Thatch to your compost bin right from the start and try to keep it there.
As for the droppings, the number you need is partly based on the size. Your best bet is to put in 200 Thatch (2 stacks) and then fill the remaining slots except for one with droppings. You can even fill that last slot, but sometimes you can hit a bug/glitch, where the fertilizer won't be produced because there is no spare storage spot in the compost bin.
Using Water Reservoirs to irrigate a farm is a daunting task. They don't hold much water, and are very slow to fill if you are relying on rainfall. You'll need quite a few for even a small farm if you aren't using a water intake pipe.
Do keep in mind that the water reservoir must be placed on a pipe and that the only way to use its water is by attaching taps to the pipes connected to the reservoir.
For a farm that is completely dependant on water resivoirs you'll want at least 4 to get a start. And you should wait until the resivoirs and crop plots are full of water before planting.