Your rendition of clothing is a powerful tool for designing a portrait beautifully. It can add a great deal to the feeling of reality you wish to achieve.
Clothing is just fabric lying flat until someone puts it on the body.
For all of us, our clothing means excess fabric, enough so we can move - bend, sit, crouch, walk. Since you will be painting many types of fabric, it's important to understand the individual characteristics of each one. The list goes on and on. It's important to really notice the fabric in the clothing of people you see around you and study the differences. Mannequins in store windows and photographs in fashion and sports magazines are good sources for study.