This is my guide to leveling a human in cata classic to get to lvl. 40.
1-5__________Northshire- Starting area. Do all quests here.
5-~10________Elwynn Forest- You don't have to finish all quests, just level enough to get to westfall.
10-~18_______Westfall- Try to finish most or all quests here.
18-~22_______Redridge Mountains- Do the quests until you get to the camoflage suit part.
22-25________Duskwood- Do the quests until you hit level 25.
25-40________Northern Stranglethorn and Stranglethorn Vale- yep.