Since you're concerned about addons, any addon you acquire from websites such as curse.com, wowinterface.com, etc will be permissible by Blizzard. I agree with Nate that you should stay away from map-revealing ones.

As a newbie, I would caution you to keep your addon selection fairly basic, and only increase use of them as you become more familiar with the game.

I disagree with Nate about Bartender (and action bar mods in general) for two reasons. First, action bar mods can be a pain in the ass to set up properly and may even require you to re-map all of your keys. Second, action bar mods are very susceptible to "breaking" whenever the game gets patched: with another expansion around the corner you'll be a lot better off without having to worry about it. While you will eventually find action bar space at a premium (especially as a mage), it will not be a problem as you start out so retaining the default setup is one less headache.

Addons I would encourage you to install, are:

X-Perl Unit Frames - replaces default unit frames with easily customizable unit frames: just click the white button on the rim of your minimap to open up the options GUI.

Gatherer - tracks locations of herbs and minerals for your gathering professions if you decide to take a gathering profession.

Addons you will find useful, but don't need, are:

Questhelper - the name says it all. It tracks your quests on the map, and shows an arrow on your minimap for where your next one is. If you have a kill or collection quest, it will show on your map the entire area where the specific items are. An improvement on the default one that's now built into the game.

Gnosis - casting bar replacement. A bit confusing to set up, but very pretty, and it has a "lag bar" when you can start casting your next spell even when your avatar looks like it's still casting the current one.

MOST IMPORTANTLY:

Read the mage class forum. Look for FAQs and build guides, typically stickied at the top. They will recommend particular mods, spell rotations, and the types of gear you should be equipping (and why). Many will pertain to end-game, but there may be some leveling stuff in there to.

Also, add friends to ask questions of. WoW is a huge game, and it will take a long time to learn what's important and really integrate everything and be comfortable with it.