Short mage raiding guide for starters
First I will say that this guide is made for new mage raiders.
The first thing you will need to do is to work with your gear right from the start, nowadays you can get great pve items from badge of justice which drops in both heroics and raids. This, aswell as the new guardian's items from pvp and you allready have a good start on your gear. Your gonna try to get high spelldmg on your mage, aswell as spellhit is nice. 16% is the spellhit cap with Elemental Precision you need 13%.
There is different speccs you can use, my favorite is Talent Calculator - World of Warcraft , many mages like to go for icy veins instead of clearcasting for the improved damage. For starters who will do karazhan and heroics you dont need to specc fire, you can go frost aswell which many people prefer.
Your main damage will be from fireball, allthough having fully stacked up scorch at all times is key since it adds alot of dmg. There is also alot of consumables you can bring to the raid to increase your dps, such as Blackened Basilisk - Item - World of Warcraft - Flask of Pure Death - Item - World of Warcraft aswell as wizard oil on weapon. You can also get Flame Cap - Item - World of Warcraft (not necessary).
Your main job is damage, providing raid with consumables, buffing aswell as crowd controlling (polimorph).
The addons I use is omen threatmeter, there is several other addons out there who can make it easyer but this is all I need.
Thats the very basics, hope it helped someone out there!