I'd suggest gathering plants at the start. It's a nice way to feed 7 Dwarves, bring some variety of booze into your fort (if you brew the stuff) and it keeps your farmers busy while your miners are digging out your first rooms.
Irrigation is tricky business. I usually start by digging out a large chamber and connecting it to the river, thus creating a source of underground water that never freezes and does not have carp! Since I'm fancy, I tend to use a flood gate and a pressure plate for this: pressure plate into the large chamber, set it up so it closes the floodgate when the water level is above 5.
Irrigation is easy beyond that: just dig a straight channel out from this large room, then dig a hole into it that leads to your would-be farm. Close the hole with a floodgate; let the channel fill with water, close the floodgate to the chamber, open the floodgate to your farm and voilá, instant muddy soil!
Of course I flooded half a dozen fortresses before I managed to get this right, but it works fine now. It not only provides irrigation, but also a safe source of water that I can channel deeper later on as well.
As an alternative method, I heard that if you dig a hole and channel it out from above, you can designate it as a pit/pond and have your dwarves haul water there with buckets. I've yet to try this, but it seems like a neat way to do it.