When leveling a shadow priest, following a few simple guidelines will keep you melting through mobs with little to no down time.
We'll start with spec. The key talents you want to pick up are:
starting in the Discipline tree,
Twin Disciplines - increase to all damage and healing
Improved Inner Fire - boost armor and spell power
Meditation - mana regen while casting
Mental Agility - makes all instant cast damage & heals cheaper
moving to the Shadow tree,
Spirit Tap + Improved - super mana regen
Everything else is up to you based on play style and what other things you do as you level.
The basic cast cycle is Mind Blast, SW: Death, SW: Pain, wand til dead.
Recast PW: Shield while the Spirit Tap regen is running, move to next target.
If SW: Pain is running out before a mob dies add Mind Flays as needed to close the gap.
Mind Blast, SW: Death, SW: Pain, Mind Flay(x) wand til dead.
Prioritize gear with high Spirit and Spell Power as you will want the most regen you can get. Health shouldn't be an issue as you will rarely take much damage.
When fighting 2 mobs or more focus on getting the SW: Pain on all mobs. Then try to get Vampiric Embrace up on as many mobs as possible. Throughout keep PW: Shield up and if it goes down early or you need to cast a real heal use fear.
