Thanks everybody for the answers.
Now that the trade problem is settled, I'd like to explain about food and drink :
Yep, I'm farming, and there's a farmer working (I manage jobs, that's not a problem). I also pick up all the plants I can from the surface (when there are plants, of course. In the mountain area I was speaking about, there were none to speak of). That's why I was farming under, and beekeeping above. And fishing and hunting, but with very limited success...
So, I'd like to know :
1) How come that I get so few from farming ? I worked with plump helmets all year round (as said in the tutorial), but I would have expected to get more and more seeds, soon enough to fill a full biggest-size plow spot with them. But, usually, even after a year, I had about the same ammount that I had at start (like, 7 or 8 spots being cultivated, the rest of my field staying blank)
2) Should I transform the food ? Like, brew it, or "extract" from it (dunno what it does), or "process it to containers" (then again, no idea what it's for. Is it merely to put in jugs, crates, barrels, sacks, and the like, to save storing space / help trading) ?
3) I had some cattle die of starvation. How do I prevent this ? They were standing on "dirt" tiles (maybe clay, maybe rock, dunno, but no grass to graze on), while there were grassy tiles a bit away (yup, grass was about the only vegetal thing available on that damn mountain). I tried putting an "animal" storage area, to no avail.
4) Those dead cattle, how do I butcher them for food ? Is it possible, or do I have to slaughter them MYSELF in order to be able to chop them to bits ? My people merely moved the corpses to the Refuse storage, while I had a butcher's workshop with "butcher dead animal" active (on repeat).
5) As for fish, I sometimes got one (a fish out of the water), but nobody would pick it up. Do I have to have, first, a fisher's workshop with "butcher fish" (or whatever it's called) activated, so that they understand it's edible ?
6) As for hunting, what is the difference between hunting animals and "capturing" them ? Is the first one easier to do ? Ain't the second option better, like keeping it either to breed, milk, shave for wool, or butcher only if needed ? How do I work it out ?
As you can see, those points aren't really addressed in the tutorial (I don't play with the pre-set savegame they tell to use, so it's not as easy)
Also, as for aquifers, I like them for the water they provide, but.. I have to say I yet have to manage at breaking through one. My latest fortress, very young, is built nearly on the aquifer (top level on map is 3 above aquifer, I can dig 2 levels (of red sand and fire clay) and then I'm hitting water. So, I'm trying to pump and setup wooden walls, but I'm not sure I'll succeed in breaking the layer.
So, about that, is a "full" block pumpable, or do I have to make it a slope for it to work ? I read the wiki pump page, and the one on aquifers, but I have to say it's still kinda obscure to me, how to pump that water successfully.
Also, if I successfully build a wall on the aquifer level, how will my carpenter (yup, wooden wall, I've got no stone) come up out of the hole ? Do I have to keep an unbuilt slope near the edge of the wall ?