Chances are you came to this guide because you were feeling discouraged about leveling your shiny new priest in World of Warcraft. Before you go any farther, stop. If your character is below level thirty and is not a Blood Elf or Night Elf, reroll to one of those characters. Those two classes have by far the best racial talents (and in my opinion are the best-looking) to be a Priest.
Now, if you are rolling a Priest just to be a Shadow Priest and nothing else (mainly to PvP) stop here now. Druids are almost as good healers in Player versus Player since they are fairly decent at Heals over Time. Although, if you want to PvE heal, roll a Priest. That being said, priests are terribly annoying to level. If you pull three or more mobs you WILL die. It takes about 15 seconds of preparing before each mob and you pretty much have to drink after every mob. If you pull two mobs you will have a difficult fight on your hands, so try to avoid it where possible. Since we are terrible at this 2+ mobs pulling questing is not usually for us. Instead, we have to grind our way to 80 /sigh. So leveling pretty much sucks hard face for us Priests. Don’t worry little young ones, even though leveling stinks, healing stinks, you don’t get any credit, get yelled at whenever anything goes wrong, and you are terrible at PvP (whilst being one-hit by every class), you still have Mind Control! Does mind Control have to be one of my favorite things about World of Warcraft? Your DPS talking smack near the meeting stone? Mind Control him or her and run that sucker off a cliff!
Anyways, as I stated before as a Priest (unless you are in a group) will pretty much need to grind instead of the quest. If you pick up skinning this really helps you while leveling against beasts. If you don’t feel like picking up the Skinning Profession, then you should grind on humanoids. This is because usually on average, humanoids drop better materials, equipment, and drop money. Also, you can get cloth from them which are very valuable.
Basically, you want to stick to large, open areas while leveling your Priest, since you can see far and wide. This ensures that you will pull as few mobs as possible. Your best bet, though is to find a leveling partner and grind with him or her. Or, better yet, get a group of friends from real life together and level with them!