This is pretty much the go to spec for mages, and I made a
video of how to set up the
Karuul Alert addon so that you can play the spec as a priority based rotation, which nets fairly decent DPS.
Defilemancer is the other spec, you can find a guide on the official forums under Classes > Mages > Mage Guides, but it's a bitch to play and only nets about 300 more DPS.
Farm Greater Marks of Ascension until you have four pieces of Hammerknell tier gear (Nyx's gear) and get the Pyromancer Synergy Crystal for a huge DPS boost. Then swap to farming Infernal Mark of Ascension and get Infernal Dawn tier gear (Infernal gear).
As for ISS farming get a DPS sigil first, then if you are in a guild who raids mostly Hammerknell work on Water Resist. If you're pro and/or being carried through Infernal Dawn, work on Fire Resist first, then Earth Resist. Resist sigils do not have to be best in slot to begin with so don't sit about waiting until you have the perfect essences, just get your sigil filled and get raiding. Although it's probably about 2000 ISS all told for a DPS sigil and every single resist, you can easily spread that over a month or three (depending on how fast you progress), and farming 200 ISS per day is perfectly feasible on a medium or busy server. Make sure you do the Instant Adventure/Rift Raid dailies and Invasion weekly for bonus ISS.
Finally, this
here is the mage gear list, updated at the moment by me. It is your bible, and will help you prepare