When you are leveling up your World of Warcraft character, there are only two things for you to consider: intellect and stamina. When you reach Outland and Northrend, you'll really start getting considerable gear that add a lot to your strike rating and damage, but you won't really need to care. Just take whatever gear is better for the long run. If you have the chance, pick up any instance gear, but a mage is not gear dependant. Not until higher levels, more armor or bigger staves are not going to compensate any lack of magical potency you have.

Gear will mostly bring about a little bit of damage, more health, and a few neat additions that help a little. Trinkets are often rare to find outside of engineering until 60 so don't put too much attention to them. Purchasing gear from the Auction House is a general no-no. Not only are instances always the way (if you can get someone to run you through them) but since your damage is magical there is no need to get a high DPS weapon and a few points of Intellect aren't going to make you an all-star grinder. At level 80, of course, all of your gear will come from instances or PvP so keep that in mind.

Mages are lucky because there isn't a whole lot they need to look for. Some classes have to balance nearly every conceivable statistic in the game all at once into their gear. Mages on the other hand just need a few things. Intellect, Stamina, Mana per 5/sec, Spell Hit, Resilience, critical strike rating and spell damage. How much of each depends on each player's taste, too. It's really subjective with no clear winners. You can buy weapons with your World of Warcraft gold. 

Intellect is needed for spell damage. It comes standard on almost every piece of cloth and weapon that a mage will desire. Stamina is as close to defense as you can get from items and you will want to pile it on to make up for the lack of armor and defense.

Critical strike rating and spell damage are all up to the player and what they wish to focus on for their build. You can get and load up on critical strike rating and focus on getting most of your damage from the Crits or you can focus on damage and increase your constant damage. Either way, you'll be getting a lot from your gear anyway.

If you absolutely must be told what to get then get critical strike rating and stamina for PvP and then get spell damage and intellect for PvE. A good judge of what to get for the current trend for both is the set items available. You'll want critical strike rating and stamina for PvP because you'll want to be able to soak up enough damage to live and do as much burst damage as possible. For PvE things are a little bit different. You want as much mana, mana regens, and damage as you can get. Again, this is subjective and based on what builds you want to use.