1. What is the deal with my cleric's religion?
I just may be the best prepared DM around for this question. I have studied a wide variety of religions and have based the dominant religion of my main campaign area on the religion of the ancient Celts, based on archaeological evidence and primary source information with the gaps filled in from "sister" Indo-European religious practices ranging from ancient Rome to Hinduism. Non-dominant religions in the primary Garnia campaign area include Celtic Christianity, Anglo-Saxon Heathenry (reconstructed in a similar manner, leaning heavier on Germanic sources), some Turkic animist/shamanist beliefs and, of course, purely fantastic demi-human religions; mostly just the pantheons taken directly from old Dragon magazines. Other areas of the campaign world have other real world religions that have mostly not come into play.
2. Where can we go to buy standard equipment?
When you create your character you can buy it out of the book at standard prices, after character creation "standard" equipment gets a little harder to find, I give a random roll based on the item you're looking for and the size of the village, town or city that you are in. Obviously the more common the item and the more populous the area, the more likely you are to find it. Then, the price of the item will be based on the same criteria plus a reaction roll with the merchant.
3. Where can we go to get platemail custom fitted for this monster I just befriended?
The short answer is no where, but if you insist, look to a large metropolis with a lot of expert armorers, expect to pay through the nose and hope for a good reaction roll. Most armorers aren't in the practice of armoring the enemies of mankind, regardless of what you might say this beast is.
4. Who is the mightiest wizard in the land?
Tough call there. Which land? In Garnia proper there are certainly some powerful wizards, mostly working out of the royal court like Merlin at Camelot. In Garnia the ability to become a wizard is genetic and not everyone with the right bloodline wants to do it, nor is every wizard created equal. The Witch-Kings of Frodia are certainly a very powerful cabal of wizards and they maintain an academy to train younger generations. The Necromancer is probably the most powerful sorcerous force that the world has ever seen, but he has been locked in magical slumber (some say frozen in death) for over a millennium.
5. Who is the greatest warrior in the land?
Most people would agree that it is the once dishonorable mercenary captain, now noble lord of the Orcish march Erc Mac Cai.
6. Who is the richest person in the land?
That honor would go to the miserly chief of the union of trading guilds of the free city of Castra Alba, whomever that honor belongs to, usually a member of the UI'Mael family. They control all trade between Garnia and Frodia, as well as having a stranglehold on the river traffic along the Avergwyn and the Averyraen, which includes all trade with the Dwarven kingdoms.
7. Where can we go to get some magical healing?
Most Temples in cities have priests capable of performing magical healing. Some towns have sacred springs or wells that can help too.
8. Where can we go to get cures for the following conditions: poison, disease, curse, level drain, lycanthropy, polymorph, alignment change, death, undeath?
This is where it gets trickier. You need much higher powered priests to do this sort of magic and they are either going to be located at major temples and holy sites or at priestly colleges. Regardless of where you find them, there is likely to be something more involved in getting your magical mojo done than just a monetary donation (although there will probably be that too), think Joel Rosenberg's Guardians of the Flame series and scale the quest to the party's need.
9. Is there a magic guild my MU belongs to or that I can join in order to get more spells?
Not so much an organized guild as a master/student relationship and an informal alliance of like minded wizards. A friend of your master is likely to help you out; a friend of a friend, maybe with a letter of recommendation; a rival of your master not too likely without a good reason. They also tend to break on a Good/Evil axis and evil magic or witchcraft (maleficium) is illegal and strictly enforced. Good wizards usually have to watch themselves, because popular myth tends to tell the story of their corruption by power.
10. Where can I find an alchemist, sage or other expert NPC?
Large cities for the most part. Occasionally with lesser nobles or in "adventuring" towns.
11. Where can I hire mercenaries?
Lots of places. Companies of mercenaries can be found sniffing around looking for hires in cities and sometimes the courts of nobles.
12. Is there any place on the map where swords are illegal, magic is outlawed or any other notable hassles from Johnny Law?
Depends on which map we're looking at. Garnia usually doesn't have any rules about carrying weapons, although you might get hassled for having weapons "above" your station, and most cities don't want to see armored men wandering about unless they wear the livery of their town guard. That's fine, nobody likes to wear armor all the time anyway, players just need to be reminded of that point of realism from time to time.
13. Which way to the nearest tavern?
It's usually just ahead on the right. Seriously.
14. What monsters are terrorizing the countryside sufficiently that if I kill them I will become famous?
Orcs. Seriously, they breed like flies and are always on the move. They have the ability to level like Humans and a "standard" Orc is tougher than a "standard" Human. They have a secure, fortified home base that we Humans have not been able to drive them out of since they took it from us centuries ago. They can interbreed with us and plant spies among us. They are smart enough to use tougher evil races for their own ends.
15. Are there any wars brewing I could go fight?
There's always a war somewhere. Most of them don't amount to much more than a feud and maybe produce a corpse or two before they get settled, but some get downright deadly. Major wars between rival nations are pretty scarce though.
16. How about gladiatorial arenas complete with hard-won glory and fabulous cash prizes?
There's always going to be someplace where there are people willing to pay money to watch men fight, whether it's to the death or not. That said, it doesn't always have to be a seedy underworld thing either, there are plenty of festivals and tournaments where you can test your skills for cash and glory, but a full on Roman arena gladiatorial combat thing, not here, sorry.
17. Are there any secret societies with sinister agendas I could join and/or fight?
It really wouldn't be much of a secret if I told you now would it?
18. What is there to eat around here?
Mostly beef and dairy products. Mutton is also common, as is pork. Near the sea seafood is also common. All of this is supplemented by hunting, particularly in noble households. For vegetable produce the common grains are wheat, oats, barley and rye; flax is also grown but mainly for the production of linen. Other common vegetables are turnips, onions, carrots and peas. Fruits and berries include apples, pears, peaches, plums, strawberries, blueberries, blackberries, raspberries and grapes. Most houses also keep an herb garden for their own kitchens. Beef and Wild Boar are the highest status foods. Domestic pork and shellfish the lowest. Usually the higher your status the more protein your diet includes.
19. Any legendary lost treasures I could be looking for?
Like the Axe of the Dwarvish Lords or the Rod of Seven Parts or the Wand of Orcus? I have a couple of things adapted to my campaign world, but mostly used the DMG as a template when I was designing, so I guess this part could use some sprucing up.
20. Where is the nearest dragon or other monster with Type H treasure?
Does anything other than a Dragon have type H treasure? Anyway, Dragons are rare and wonderful creatures that are generally far from civilized areas. Seek them hundreds, if not thousands of miles from the nearest lands settled by men or in the deepest levels of ancient megadungeons.