First if you do not know how to install a addon go here-HOW TO INSTALL ADDONS | World of Warcraft @ Curse.com
--The leveling guide--
WoWInterface Downloads : Tekkub's Emporium of Random Crap : TourGuide
its the best in game guide i have seen.What it does it give you a good leveling pattern for the horde or alliance.The horde guide is pretty much a joana rip off but think of joanas guide in game...The ally guide was recently rewriten and have not tested it yet but i will begin my ally journey soon
There is a addon that will improve it tho called TomTom
WoWInterface Downloads : Map Mods : TomTom
what this does is all the coords the TourGuide has it will put a arrow on youre mini map that shows you the way to go it also puts it on youre main map.
Other addons i recomend.
AutoProfitX- sells all the grey items in you're inventory.(good if you are racing to 70)
AutoProfitX - Inventory/Item - World of Warcraft Mods, Addons, and More!
Informat package from auctionneer
Auctioneer addon for World of Warcraft
This shows you how much a item sells for and what it is used in.
Thats all i use when i am racing to 70 this might be a repost but hopefully my other addons will be news to you.
NOTE::: These addons update alot....make sure you check and see if they updated ever couple of day.
and if you know anymore addons that will help you level please tell me i like new addons )give link and tell what it does would be useful to)