Now disc stat weights, like those of most healing specs, are not set completely in stone. Some stats are better than others, some are pretty much useless (spirit, I'm looking at you, get the gorram heck off my healing tier), but how you weigh them does depend at least somewhat on your personal style and preference.
So instead of just rewording what Elitist Jerks has to say on the subject, I figured I'd tell you a) what all the various casting stats do for you, and b) what I do . That way you can make a more informed decision.
First, stats you don't want if possible...
Spirit. This does very little for you. Seriously. It's on healing tier cause holy likes it and because Blizzard seems to like pushing it. It's like intellect's much more annoying little brother whose mom is determined to convince everyone is cute, not annoying. Or something. Not sure where I was going with that, but spirit, not worth it. What it does for you is 0.365 mp5 per point. Mp5 is shorthand for restores X mana every 5 seconds. Other stats give you much much more. Heck, straight mp5 is better imho.
To hit. To hit is for dps. Your heals will hit friendly targets, period. Sometimes good upgrades will have to hit on them because well, dps casters like the same stats we do. This may lead to headed arguments, which often include gems like "Healers like spirit, not to hit!" You don't like to hit, but heck trees don't like crit and yet they often end up with gear with crit on it. Occasionally you have to put up with a stat you don't like or don't care about to get more of the stats you do care about.
Second, stats that are awesome and shiny...
Intellect. You love this stat. Trust me. Intellect gives you 0.817 mp5 per point, ~0.0076 crit per point, and increases the size of your mana pool. Since most disc priests don't tend to run with the regen holy priests do, a large mana pool is a must. As a new 80, I'd even suggest gemming for int to begin with, and switching to other gems as you gain more intellect on your gear. Basically as you stop running out of mana, start switching over. Currently I run with roughly 30k buffed, and that is more than enough unless I'm shielding like there is no tomorrow, in a long fight, while constantly casting. How big you want your mana pool to be is a personal thing, so kinda feel it out. If you have a regen trinket like Solace, you need less int, and if you're constantly casting and dealing with high-damage fights, you need more. It really does depend on what you're doing and how you're doing it.
Spellpower. This is your favorite stat after intellect, or possibly before intellect. Spellpower makes your heals more powerful, and thus makes you more powerful. Power Word:Shield's absorption also becomes better the more spellpower you have. Best stat ever! Yes, I'm biased.
Crit. This is a nice stat, but how much of it you want does depend on what you're doing. Crit makes your spells more likely to critically hit, which means they hit with extra power. If you're raid healing with PW:S and Prayer of Mending, crits are only possible with the heal from the glyph of PW:S and Mending. But if you're tank healing, or throwing flash heals around like there's no tomorrow, well crits start looking much better. Divine Aegis depends on us critting though, so I certainly never say no to this stat. EJ recommends stacking crit once you've reached your haste cap, and while I personally don't do that (mmm, yummy spellpower), it's probably not a bad idea.
Haste. Haste makes you cast faster. It decreases your global cooldown as well, which makes even instant casts faster. Your minimum global cooldown is 1 sec, so once you hit a certain point of haste it's not doing you as much good as some of the other stats could be. Assuming the following raid buffs (Wraith of Air totem for 5% haste, and moonkin aura/Swift Retribution for another 3%) and our own talent Enlightment for 6% haste, and you really don't need much more to reach the hard haste cap for PW:S casting. Basically, if you want all your PW:S casts to be as fast as possible, you need another 154 haste (or roughly 11% haste). Haste can still speed up your longer casts even after it's no longer any good for PW:S, and the answer of when you stop wanting it is really complicated. So complicated, in fact, that if you're really interested, I suggest you go read the value of haste on EJ. I personally am quite happy with my current haste stat (451, way over cap) and would not go out of my way to increase it.
So those are the stats you're likely to see. There's a minor one you might run into, straight mp5, and I'd say take that over spirit. But otherwise, yup those are your stats. Now, the idea here is for you to read this and figure out what you think you need, but just in case you want a simple this is what you should do, I give you...
How I gear, stats and gems...
Spellpower trumps everything else. Spellpower is shiny awesomesauce as far as I am concerned.
More intellect is yummy too.
Haste and crit are nice, but I don't fret about them.
So my stat weighing looks like SP>int>haste+crit>>spirit and to hit.
I gem mostly for spellpower (runed cardinal rubies and whatever the sp dragon's eyes are called) with some other colors for meta activation. I use the 21 int+ mana regen meta, and I ignore socket bonuses unless a) I need one for my meta or b) the socket gives me more spellpower than I lose by not gemming straight spellpower. In fact, my gemming is kinda messy right now, and I should clean that up. I know why I have an int/sp gem, but why one sp/spirit and one sp/mp5 is beyond me. (looks more closely...)
Okay, the sp/mp5 was apparently for a spellpower socket bonus, and the other two are for meta activation. Nice to know there was some logic there.
Now if you want a list of gear you should get, exactly what enchants, gems, ect, well, see the first link? There's a really good place to start. Heck, they even suggest buff foods. Granted, they don't lay out exactly what gear a new priest might want, but honestly, run dungeons, run heroics, go by the stat weighings, and you'll end up decently geared. Might even want to get your hands on full tier 9. I had it for a while, before I replaced it with other stuff with better stats, but I 've still got the two piece bonus. And I'm still rocking a badge trinket (all that int is not really useful to me anymore but as much as I want a heroic Solace I'm unlikely to see one any time soon), with Nevermelting Ice Crystal in my bags for when I'm tank healing and want more crit.
And if you're still completely at sea, you can always email me to ask questions, hit the plus heal forums, or hit wowladies over on livejournal. There's a ton of resources out there, and I am really just scratching the surface of this entire subject.