Interfacing is used to add stability to a garment and improve the drape.  It is usually used down the front of a jacket or shirt, in collars (all parts), shirt cuffs, and waistbands.  Use a weight that is compatible with the fabric you are using in your garment - the packaging will have this information.  In a quilted garment, the batting and quilting stitches add the stability, so you don't need interfacing.