Tips:

Get one gathering profession, and get skinning.

Get First Aid, IT WILL HELP. The four exceptions to this are, of course, the four classes that can heal, Priest, Paladin, Druid, or Shaman.

Buy armour off merchants and/or auction house. People say not too, they say it wastes requiem lant, but especially armour reliant classes like Warriors and Rogues need new armour almost every five levels, buy the cheapest armour that will still be useful to you.

Specc your talents in the tree that will do the most damage.
-Druids: Feral

-Hunters: Beast Mastery or MarksMan

-Mage: Fire or frost

-Paladin: Retribution (this talent tree is best for leveling, and arguably nothing else)

-Priest: Shadow

-Rogue: Combat or Assassination

-Shaman: Elemental AND Enhancement

-Warlock: Affliction (A lot of people say Demonology is better, I personally love Affliction much more but if you feel the need to try them both go ahead)

-Warrior: Arms or Fury

Always get requiem lant quests related to what you are grinding (i.e., kill X amount of Y, or gather X off of Y), hey, you're grinding them anyway, why not get an extra EXP boost?

Listen to music. Grinding may be quick in most aspects, but it's BORING. Have something to keep your mind off of it. Listening to comedians are good too.

If you have the time, run and instance! Go into LFG when you begin grinding, when your group is full stop grinding and run the instance, go back to grinding afterwards. I would not, however, suggest running nothing but instances, limit yourself to one or two times a day.

Don't turn in quests the second you finish them, wait 'til you're done grinding in that area to turn it in.

ALWAYS ALWAYS Log out at an inn, the rested exp adds up.

Don't change your Hearthstone location unless I ask you too.

The Guide

Durotar:

Levels 1-6:

This guide starts in the TROLL and ORC starting area. Meaning if you aren't either of those two races you will not start here. Either come here or quest/grind in your own starting area.

1: Standing in front of you there is an NPC named Kaltunk. Get "Your requiem lant Place in the World" from him. Go into the big cavelike building, and turn in "Your Place in the World" to Gornek. Get the follow-up "Cutting Teeth" from him. Go outside and to the left, there are a surplus of Mottled Boar to kill. Kill them until you have reached level four and have completed "Cutting Teeth."