if you are using the icy veins class guide rotation and talents and watching videos, and still performing poorly it could be other things. Are you using a MMO mouse with 12 side buttons so you never have to stop moving to cast instant skills? Read this thread i showed how to make some basic weak auras to improve reaction time. if all else fails, it might be that you have slow reaction time. Fire mage needs fast reaction time more than most classes.
https://us.forums.blizzard.com/en/wow/t/new-player-need-helptipsadvice/783386/9