There's a "blog" called wowinsder.com that could provide you with a TON of information. I imagine if you search "priest" you could find a listing of good starting gear and where to get it. But to save you some time, here are two excellent posts for both shadow and holy:
Spiritual Guidance: Hitting the cap - WoW Insider Spiritual Guidance: Easy pre-Naxx gear for Holy priests - WoW Insider.
The place to go to "l2p" as it were is probably the Elitist Jerks forums. They're very heavy on the theorycrafting over there, but if you can spend some time going through it, there's a lot of really good information to be had. They can be found here:
Elitist Jerks.
Basically you have the idea right on the sources of gear and the order of their power. "Weak" items come from questing (with the occassional gem), "Good" items come from regular instances, "Great" items come from heroic instances and "Awesome" items come from raiding. The power of the item corresponds relatively well to the difficulty of coordinating what you have to do to get the item.