First of all, sorry for the length of this. Secondly, if you’re more interested in taking it easy and learning as you go (highly reccomended), you can pretty much skip from the break to the PS. Though the stuff in between is a bunch of common sense, research, and experience that’s applicable even to a more casual player.
Lady of the Lake, heh, it’s no problem. I enjoy the game, and like to help out casual players. This is because (imho) more dedicated players tend to give somewhat misleading advice, or portray the game in a specific light, and I feel that someone just starting out can get a bad impression if they don’t hear from a broader range of perspectives.
Well, as for talents, it’s not a big deal really. At the early levels, nothing’s going to make that big a difference, so if you just spend the points where they interest you, it’ll be fine. It costs a bit to respec (reassign your talents, it’s done at any class trainer, and gets more expensive each time you do it), but you should be able to easily afford the first time during your 20’s, and should have a better idea what you want then too.
…break…
If you do want to research it, I highly recommend WoWwiki’s talent, build and abilities pages for any given class. Great source for concise information on the actual gameplay mechanics. Another site, which I find has more helpful commentary, is WoWhead (click on browse, and navigate the drop down menus, the page for each talent or ability has a user commentary thread). I’ve found both to be much easier to get a clear answer from than the official forums.
To give you a good general perception, each class has three trees, that usually emphasize different aspects of the class. Each tree has talents at different “tiers,” and in order to progress down to a given tier you need to have spent a certain number of points in that tree. When you reach level 10, an arrowhead icon will appear in your user interface in the row with your character sheet, questbook, spellbook, settings, etc, buttons. This brings up the single window you use for managing talents. Most talents will have different degrees of effect, requiring you to spend more than one talent point in them to get the most out of them. Some talents will be linked to others (this is shown graphically on the tree, and explicitly in the tooltip textbubble that shows up when you point to the icon), requiring you to know one before you can learn the other. Generally speaking, it’s better to “buy out” a talent if it has more than one degree, and it’s usually better to focus on one tree at a time (furthermore, most successfull builds only invest in one or two trees, though I break that rule often when I play around with planning out my talents 
).
Reading the talents’ descriptions, along with the tree’s name, will usually give you a good idea what attributes they would gear your character for. Many classes have separate builds that excel in either PvP, Questing (solo or groups), or raiding. If you’re interested in progressing to the higher levels (I personally haven’t felt much drive for that since I deleted my first guy, a level 40-something Paladin) I would advise pursuing a questing build (or better yet, a “grinding” build, if you can stand the monotony).
IIRC, Priests’ trees are Holy, Discipline, and Shadow. Holy is all about healing and support spells (focus on grouping early on, and raiding later). Shadow is about damage (PVP and solo questing). Discipline didn’t strike me as having any particular application, but rather it seemed to improve a priest overall, making them more efficient, more durable, etc. Most Priest builds I’ve seen that aren’t expressly for the end-game player (hardcore pvp’er or raiders) hybridize Discipline with one of the other two (in a 2/3 ratio, Disc. being the lesser).
PS. Early on you mentioned seeing Priests who where glowy and transparent, right? That’s Shadowform, a high-end Shadow talent that gives the Priest a high level of resistance to physical damage, but prevents the use of Holy spells (most of your healing spells). It’s very popular with PVP Priests, and raiding Priests who want to be Mages. 
One of the positives to Vent is that we can still communicate in the event of server problems. We’ve had a couple of raids when the instance server went down.
