Typical solo Shaman leveling is Enhancement to 39/40 then either keep going as Enhance or re-spec into Elemental once you have Lightning Mastery available.

How are you planning on leveling? Just remember that without a Mage in group doing Elemental from any level is going to be water intensive, and as a result probably quite expensive. A melee team with a healer I imagine would work quite well from level 30 onward which is when you pick up Windfury Weapon.