You will actually go through several phases in the 'type' of bonuses you wish.
Early, you may want school specific that confer +x damage or accuracy bonus. Health adds are always good.
Once you start throwing spells with damage over 100, you will probably want to switch to clothing with a +y% bonus, again to your school. Look for health adds.
As you start encountering higher level foes, you may wish to go to clothing that gives defense vs all schools (usually you have to sell goods to get these items). By this time you might have items that gives cards (a necklace that gives a 2-pip attack), so most of these multi-school clothes will boost this damage also.
Again, like Gamma stated, look at your school, Storm wants to have accuracy bonuses and health, Ice wants damage bonuses, etc, and select modifiers that you feel would enhance your play style.