A few times I had buildings stall during constructions. (a) One time the announcements mentioned the area was not clear. Is that a bush, stone, or other obstacle and how can I clear it?
(a) More accurately, the problem here is usually that one of the objects blocking that blueprint has been reserved for some other project. Since it has been claimed for a job, it cannot be used for other jobs (like being picked up and moved out of the way). Just give your other projects some time and try again later. If there was an obstruction that you needed a separate command to clear, it wouldn't have let you place it to begin with.
(b) Another time a carpentry workshop stalled just waying "carpentry" required. I built another one and canceled the original. Was that likely some obstacle in the way or do the dwarves give up on buildings that have some temporary obstruction (skills, needed things of other kinds)?
(b) This means that only someone with the Carpentry labor preference enabled can do that work. If it is taking too long to get done, that means your carpenter dwarves have a lot of other jobs they have to do. Untangling this sort of issue so that all your tasks are getting done at the pace you want is the core gameplay.
Pick someone with that labor enabled, and (f)ollow them around to get an idea of what they're doing. This is a good example of why an app like Dwarf Therapist is useful. If you have that app installed, you can use it to quickly disable all other labors for a particular dwarf, so that it is only doing Carpentry jobs, wait for it to finally build that workshop, and then re-enable all the labors they previously had on. Trying to do this through the base interface of the game is quite cumbersome.
Note that skill will only affect how long it takes to do those tasks. A dwarf with the labor enabled will successfully do any task you set it to, eventually.
Labor and assigning labor preferences is a complex subject. You can find out more on the wiki:
https://dwarffortresswiki.org/index.php/DF2014:Labor What should I think about when making staircases down?
My staircase was like (all built using "down" stairs). I assumed "down" works for "up" too).
Here is a great video to explain this:
https://youtu.be/x_21Y34Yx4cMany people build more than 1 tile of stairs for the same reason that they build wide hallways -- it increases how many dwarves can move through that path without bumping into each other. Dwarves CAN bump into each other, knocking one of them over, and falling to the bottom of the staircase (unless stopped by a floor hatch).
More than that, though, take some time to think about the design of your fortress and the utility of what you're building. Barring any !!FUN!!, you'll eventually have up to 300 dwarves running around your fort. Layout will be important. Thinking about how you want to lay it out, defend it, and organize it, is the game. I spend more time with DF paused than I spend letting it run (though that's my playstyle, not everyone does it like that).
I tried to make a refuse/garbage tip using the same kind of rectangular selection that works OK for digging i.e. select the start square then move to the end point and select (enter) again. It didn't make a rectangular shape indicating success. Does it work in another way?
The interface is inconsistent, but yes, you should see a visual change. Since you compare it to digging, let me just just check: you're marking open floor space for that stockpile, yes?
The wiki page on stockpiles goes into this thoroughly:
https://dwarffortresswiki.org/index.php/DF2014:StockpileI had trouble growing plump helmets in a cave. Is the floor the problem? I guess I should be looking for a muddy floor? or is any floor type OK?
Farm Plots and the food industry is a pretty big topic, and the wiki pages on the topic are pretty extensive:
https://dwarffortresswiki.org/index.php/DF2014:FarmingThere are a number of steps you might have not noticed, or performed incorrectly. In summary:
1. Designate a Farm Plot on soil (somewhere marked Underground for plump helmets). Someone with Farming with build the plot.
2. (Q)uery the plot, and then designate which crops you want planted in each season.
3. Your farmer will plant Plump Helmet Spawn (ie, seeds) during the appropriate season. Any dwarf with the Food Hauling labor will harvest it once it is ready.
4. Eating the plump helmet raw or brewing it into dwarven wine will create seeds for your next planting. Cooking the plump helmets as part of a meal in the Kitchen will destroy the seed.