Before I start, you need to realize this is not a 1-70 leveling guide, it's just a short guide that will help you have a quick understanding of how to level up a mage or a warrior in the lower levels, if you need a complete leveling guide, the best ones are: Joana's 1-70 Horde leveling Guide for horde, and Brian's 1-70 Alliance leveling guide for alliance.

For a Mage I would say to "grind" is a bad idea, I was grinding (unrested) from 17-18. Even though I was taking on creatures of my level and higher it still took me 1.5 hours. Now that compared to 18-18 3/4 in about 45 min doing 3 quests (1 quests I had to get 60 tusks though). So I would say go questing for a mage, or do some instances, that is until u get improved arc explosion then u can get lots off people at the same time.

For a warrior I would encourage grinding as a warrior is one of the easiest classes to grind with. They can be a high damage (2h) high armor class. So go grinding until u can get to WC (run it for Wingblade) then go grind until u get a good 2h. Once u get to be able to do SM u should have your Whirlwind weapon so use that for taking on mobs of 1 to 2 people then def stance+ 1h shield for mobs with more than 2 people. After that I go to Arathi and Alterac Mount. In Alterac go straight up north and grind there, I got 32.5-33 in 30min and 33-34 bout 1hr, also the mobs have some good loot. In Arathi go to the circles of binding when u get to lvl 35 and use your WW weapon. For both Arathi and Alterac it helps to have heavy silk bandages and Mageweave, but food works too. Also when u get to 225 first aid u can do the quest in Arathi for up to 300 skill (takes some luck and repetition, took me 5 times to finally pass :)

And lastly I know that this is a n00b guide but I will up date as I go, as right now I am farming honor at level (wow you can get a lot of honor for killing people 8 levels ahead of you :P)