You are a rectangle if shoulders and hips are about the same width and you don't have a lot of waist definition.
Style tips for Rectangles
Create the illusion of curves by drawing attention upwards (strapless, v-neck, one shoulder) or towards the hem (Mermaid)
Define your waistline. An empire waist will showcase the smallest part of your torso. Or a beaded detail or colored sash around the natural waistline will create definition.
Skip:
Halter and High Neck: will make shoulders look narrow and as a result will not flatter your waist