Something that I have no idea if is mentioned in the Wiki is that aboveground farms suffers, sometimes, from "stray rock" syndrome. Your fields are dotted with places undesignatable as a planting area. But what I do is channel down into the soil (as with previous post, assuming here that you have a soil layers, rather than just a surface) and you get uninterrupted soil.
Underground farming can best be done alongside, in hollowed-out areas of sub-surface soil
not uncovered by channelling the ceiling (ground-level) away. And there are ways and means to make sure that even
flying pests (and enemies) cannot interrupt business, but I'm sure they're mentioned.
On the whole, it's honestly easier to make a farm in(/on) soil than to get irrigation onto bare rock. There's times when it's
easy to flood an area (e.g. easy access to an aquifer, river or pond/lake of underground or underground type) but the problem might well be, in that case, that of getting a floodgate or other barrier set up so that you don't
over-flood it, and can safely get rid of the excess water straight afterwards. [Edit: And, yup, Garath's solution works, but beware of the pool replenishing faster than you can drain the water you got from it.
]
(Just one among many of the aspects of fortress management that will doubtless become second-nature to you, in the future, but right now I'd be tempted to suggest you get the 'easy' bits done first and
then play around, once you've got a working fortress running. OTOH, there's nothing like an emergency for forcing analytical thinking.
)
AnotherEdit: Do you have buckets? To spare?