Im lvl 55 frost mage, & i want to improve my critic rate,
so, beside talents, what equipments shall I have?

grind like a mother to 58, then hit outlands, you will replace everything (and I do mean everything, armor, weapons, trinkets) you have within two or three levels with outlands greens and blues, and your intellect, +spell damage, and crit will go thru the roof.

anything you find in Azeroth should be gone by the time you hit lvl 60 (2 levels in outland)
just forget about good gear... its a destraction from killing
quest, kill, do whatever... get to 58, get to outland, get good gear
quest rewards in outland and drops will be much better