Hello. So to answer your questions...
1) go arcane (look up my arcane spec)It always does high dps. After your decently geared (264 ilvl items mostly) go Fire.
2) use my arcane glyphs for arcane spec
3) rotation is mostly the same, you cast blizzard on trash ( 3+ mobs) and boss rotation is Arcane Blast x 4, use Arcane missiles IF missile barrage procs, otherwise use Arcane Barrage. Also remember to keep slow all the time on target. I suggest you also pop up Icy Veins and Arcane power to max out your dps.
If i helped you then im happy