Don't listen to these people saying to go shadow for priest.

Get 5 points in Spirit Tap, then go Wand Specialization and continue in Discipline. You can respec shadow at 40.

Have Spirit on all of your gear.

Mind Blast>Shadow Word: Pain>Wand.

If that doesn't kill fast enough throw a Holy Fire or Smite after the Mind Blast.

This will leave you always with plenty of mana in case of trouble, and kill reasonably quickly.

In dungeons, let your party members get low enough on health that your best heal only barely tops them up without overhealing.

This will leave you plenty of time outside of the five second rule to regenerate mana with all your spirit.

Using this method I never carried water on my priest until level 45. Never needed it.