1. Usually a priest should pull with a slow casting Direct Damage spell like Smite (Mind Blast, if there no other mobs nearby the target) at the edge of the spell's range. The next spell should usually be Shadow Word: Pain, before the enemy gets within melee combat range. Because it is instant cast, you can be backing up while casting it, to extend the amount of time the mob takes to get to you.
As you get around 20+ level and if spent most on shadow talents along with Mind Flay, you can use Mind Flay to further slow your target down and give your DoT spell time to do its work.
Power Word: Shield prevents spell interruption from many forms of damage, and will help you (or another spellcaster in your party) when casting spells greatly.
2. Intellect, Spirit, Stamina. In that order.
3. Levels 10-15? Emphasis on Shadow talents. Taking down mobs fast and reducing downtime are the most important things. Your built-in healing abilities will be ample for any groups you are in. Your talents at level 20:
Talent Calculator - World of Warcraft
- Spirit Tap: Faster mana regen between mobs.
- Shadow Focus: Allows you to concentrate on mobs of a higher level than yourself.
- Improved SW:P: As the harder battles last longer than 18 seconds.
- Mind Flay: The most useful talent in the Shadow tree. It has high DPS and high mana efficiency.