Rule number # 1: find the right heels for yourself
This is not because your friend at work wear that specific hight of heels and feels comfortable in them that will be ass well. You do not necessarily have the same shape of feet.
The only solution : try lots of different models. Not necessarily in a very expensive brand . The most important is not the price of the shoes but the way it fits you. We need the heel to suits you , that's all.
Rule number # 2 : Know the size
It seems obvious. Except that sometimes it is not. Too wide , the foot will not be maintained. Your toes will look ugly (especially with sandals ) .
Rule number 3 : Do not focus on the height
A medium heel can be more uncomfortable than the very high because the camber (the angle between the heel and forefoot ) is too high. Too high forces your foot to rest on the front.