Welcome to the board. There are all types of players around here - raiders anywhere from world-ranked to classically casual, and the casual-core like me in between, some who live for PvP only, and some for whom there is no "endgame", they just keep playing the game. But an honest question, at any level, generally gets an honest answer.
As for 1&2, the seal/judgement system has changed quite a bit over time, and the in-game language describing it hasn't all caught up. When I started, there were a number of seals, but only one Judgement, and the effect of the judgement was based on the seal, and consumed the seal - i.e. the judgement "unleashed" whatever seal was up. At the moment, seals and judgements are linked in the sense that you can only judge if you are sealed, but the effects are independent. So you can mix and match on demand without the preplanning of the old days (for example, as a holy paladin, I seal wisdom for raids, but judge wisdom or light variously based on the situation).
A macro is a small script. It can do more than a simple command you type into the /chat window, but not quite as much as a full-fledged addon. Their power to make decisions is very limited, so they can't do much - you can't, say, write a macro to use Hammer of Wrath if the target is under 20% health and Exorcism otherwise. But you can do simple things like making sure that Divine Illumination is always cast immediately before a Holy Light.
Raiding is, at its simplest, just putting together a group of more than 5 people to accomplish what a party cannot. Typically, it's in the context of a group of 10 or 25 level-capped players attacking an instance like Ulduar that was designed to test a raid. But you also commonly see raid groups for PvP battlegrounds, world PvP like attacking enemy cities, or taking down an elite like a world dragon. It could even be 40 level 1 characters trying for Hogger in Elwynn Forest. As I implied above, the raiding we usually think of when we say "raiding" is generally done at the level cap, currently 80. As for how, it's organized like 5-man instancing, but larger - guilds are organized specifically around raiding, and your server will have some amount of pick-up group (PUG) raiding outside of guilds.
The best gear is found in the toughest content. That means 5-man instances and (eventually) raids. Until you get close to the level cap, though, doing instances just for the gear (as opposed to doing it for fun or for practice) isn't worth it. Anything you'd get right now from Sunken Temple, even if is an improvement currently, will be obsolete about three seconds after you hit Outland - and much the same is true for Shadow Labyrinth/Northrend ten levels from now. Do your research (i.e. the FAQ here), ask questions, learn your class/spec - learn what stats are important to you (for a ret pally, Strength, Hit, Expertise give the best bang for the buck) and which aren't (for a ret pally, Haste and Armor Penetration). When you know what you are looking for, quest rewards supplemented with cheap crafted gear and an occasional AH purchase (if it's a real good value) will do you just fine. Especially for your first character, you don't have the spare gold to buy your upgrades when they will be useful for such a short time before you'd have to buy another.
For a leveling Ret paladin, you want to concentrate on damage and survivability. This generally means Strength and Stamina - especially before Outland, where the choices open up. You won't go wrong by sticking to plate and mail with STR/STA, that will keep your health and armor up (don't worry about armor rating, stick with metal and the tiny differences won't matter). Avoid holy paladin plate and shaman mail with spellpower and intellect - that used to be good, but now you want to be geared more like a warrior or death knight. Later, the stat choices will open up quite a bit, but that won't matter much unless you want to do endgame content.