So,
Warrior: Depends on the spec you want to level up, if DPS focus on plate that gives Strength, Stamina, Hit Rating, Crit. If TANK focus on plate with Stamina, Strength, Dodge Rating, Parry rating, extra armor.
Hunter: Mainly Agility/Stamina items with Crit and haste/Hit
Priest: Choose Cloth that has Intellect, Spirit - if healer, Spell Power AND Hit Rating too if you are not going to heal!
Mage: Pick Cloth items with main stats Intellect and Stamina, also make sure they give Spell Power and/or Hit Rating for sure!
Druid: Depends on the spec again, as a Feral DPS pick up Agility/Stamina with Crit and Hit rating bonus. IF HEALER pick up Intellect/Stamina, Spirit, with Haste and/or Crit - As a Balance, pick Intellect/Stamina with Spell Power AND Hit Rating, avoid Haste/Spirit!
DK: Plate again with Strength/Stamina, Hit Rating, and crit too. If you are going to TANK, pick up the dodge rating/parry ones - always with Stamina and Strength.
Shaman: Hit mail with Intellect and Spell Power mainly, and Spirit if you are going to heal. For Melee Shaman choose agility weapons and armor - always with stamina.
Warlock: Get full Intellect/Stamina cloth with Spell Power and Hit Rating. Also Spirit helps a little.
Finally, Rogues need the leather items with Agility/Stamina and Critical with Hit Rating.
Make sure to get the Guild Heirlooms too! They are a big help! ( if you are Honored with your guild )
Also: Stamina is a main Stat and can be found on every Heirloom you are going to buy!
Hope I helped you!
Happy Gaming!!!