Structure
Try to find a soil that is not too moist. In other words try to find a soil that dries out well and does not hold water like mud. It should say on the packet whether the soil is a wet or dry one. Try to find a balance between the two. Wet soil will sometimes cause plant damage because your roots need air to breathe. Dry soil may also dry out too quickly and needs to be watered more often. Try to find a soil that is loose and feels fine but slightly heavy in your hands.
Basically the soil structure is up to you. Do not buy anything that is hard and bulky or too soft and weightless. Medium Soft and heavy is what you want.