What do they eat?
It's my favorite question of all when it comes to world building. People need an awful lot of stuff when you get right down to it, food is probably on top of the list.
It is such a huge part of any culture that it seems unwise to skip this step. There are many interesting things you can do to add *cough* a little flavor and realism.
What kind of food?
It depends on many, many factors such as climate, region, culture, population size, time period, technology, knowledge, discoveries or trade as to what kind of food they'll be able to produce or even consider.
Things like class, gender, type of work, access, difficulty to make, religion, time of year, holidays, might also play a factor in what people eat or are allowed to eat or deem appropriate.
What do your monsters eat?
Generally speaking, fantastical creatures need to eat as well and if they are offered a relatively easy source such as your livestock, they are probably getting interested. But even ignoring that, you should have a rough idea, though the more the better, how these creatures interact with your ecosystem.
You could say something like: My world only supports X dragons; their consumption is just that high. It's just that I tend to roll my eyes when I have a story with loads of giant monsters while the ecosystem looks to be perfectly unaffected by it. Those things will make a dent.
Where are the goods coming from?
Ask yourself, can your world even produce silk? Is it imported? How can a tiny country in the middle of bloody nowhere support itself, let alone get its hands on these luxury items?
It's just another tiny thing that will weaken my suspension of disbelief. That said, you don't need to lecture me for page after page as these things can be woven into the narrative and drip-fed to me over the course of a book.
Play videogames.
If you are a gamer, that is. While world building is one of the things you can learn reading and writing, I find it is very interesting to literally walk around in somebody else's to have a closer look. Fallout 3 did it differently than Fallout: New Vegas. You can look at Skyrim versus Dragon Age: Origins.