For the quest bit, sometimes it takes a little finagling on the syntax to get the NPCs to pipe up. As I'm sure you've noticed, you use the bracketed text to get an idea of what to say:
An_NPC01 says "Gee, I sure wish I knew where I dropped my [sword]."
You say "Sword?"
You say "Which sword?"
You say "What sword?"
An_NPC01 says "I had a fine sword that I dropped when I was being [chased by orcs]"
You say "When were you chased by orcs?"
You say "Why were you chased by orcs?"
An_NPC01 says "I must have gotten lost off the path in Greater Faydark, because when I got near some tents, orcs came out and..."
etc etc
It's usually not that annoying, but sometimes you have to wonder.
Mobs sometimes loiter by zonelines if trained there. Wait a few and they'll typically path back to their spawn point. By the way, the forest you were in was the home of the dark elves. You can pass through it by staying off the trails and eventually reach a zone called Lavastorm Mountains, but the paths and bridge are well patrolled by your enemies.
Simply clicking on another PC should show your their HP bar, same as if you click on a monster.
Merchants rarely sell armor worth the cost as part of their general inventory. I didn't catch what server you're on, but if you're on an established server you can usually buy better armor for cheaper in the Bazaar. The server Morden Rasp doesn't have much of an economy yet (sort of shocking to enter the Bazaar and see 60 traders and only 1075 items for sale) so that might be the exception. Anyway, no use crying over spilt milk; just don't spend money on the merchant sold chain or plate armors which are obscenely overpriced.
Some mobs are social, in that if they see you attacking their friend, they'll jump in. Most humanoid mobs (orcs, goblins, etc) are like that as well as many animals. Sometimes a mob isn't social, but naturally wants to kill you anyway. In which case, if it wanders by you fighting, it might jump in just on its own free will. Examples:
(1) If you're fighting a giant spider and another spider walks by, it'll come to help its spider-buddy. If a lion comes by, it'll attack you just because it wants to eat you and doesn't care about the spiders you're killing.
(2) On the other hand, if you're fighting a lion and another lion comes by, the second lion will jump in to help its lion-buddy. Should a giant spider then come by, it'll continue to ignore you because spiders don't attack until they're provoked.
(3) Finally, if you're attacking a snake and another snake comes by, it'll ignore you because snakes care about no one but themselves. Of course, this doesn't preclude the lion from example #1 from coming by and taking a swat at you.
Learning which mobs do what is largely a matter of experimentation, but can really come in helpful in the long run. When you're fighting a mob and see another coming near, it's good to know if you can safely ignore it, brace for an extra battle or hopefully back away out of the range of the second mob. It can be worth it to take two or three free hits from Mob A if it means getting out of range of Mob B and avoiding a 2-on-1 battle.
It's a common newbie mistake to confuse NPC and PC guilds. PC guilds are organizations of players who share some common goals, be it raiding, tradeskills or just conversation and helping one another out casually. These are the guilds you see listed above players' heads such as <Order of the Blade> or somesuch. How you join is up to the people running the guild, but there's some other threads on it and I'd rather find one of them and link it than type it all out again
NPC guilds are really just a place for you to get started with training and quests. If you're a human paladin of Mithanial Marr, you belong to the Knights of Truth. It's a good idea to stay in good standing (i.e. don't go killing NPC paladins for example) since this is your base of operations and you don't want to be killed when you go home to train your skills. As you travel, you may find other guilds partially accept you, for instance, a human warrior can train in both Freeport and Qeynos and an agnostic wizard can train at most wizard guilds across Norrath. However most temples won't train clerics or paladins from outside their order without you first proving allegiance, typically by killing hordes of monsters they're at war with.
The big exception to all this is the Plane of Knowledge. PoK has trainers of every class who'll train any schlub who walks in the door. So even if every paladin guild on the face of the earth hates you, you can always train in PoK.