To use semiotic theory to understand a Calvin Klein perfume advertisement is to treat that ad as a system of signifiers. There has been considerable scholarly work done on the semiotics of fashion since the groundbreaking work of Roland Barthes (reading Barthes would be a good starting point for your project).
First, locate the specific advertisement with a system of signification that constitutes the background langue, or meaning-creating structure. Look at how visual conventions are used to suggest actions (usually sexual) that the viewer is being asked to assume happen before or after the instant of the image. Look for the various items that signify luxury. Examine how the poses of the models are created to reference famous works of art.