I had been garment sewing for almost 10 years before I made my first quilt. Over the years I have taken a few lessons in garment sewing. I worked for 18 months for a Bernina dealer who made ends meet by doing custom dressmaking and alterations. I really learned a lot there. The most important part of garment sewing is much the same as quilting. Straight of grain is the most important factor in the success of a garment. Nothing hangs right if it is off grain. There are some wonderful books on basic sewing available probably at your local library. Nancy Zienman has lots of them. For more complicated stuff try books by Sandra Betzena [?spelling] Over the years I made my daughter's prom gown and three dresses for my grand daughters who were flower girls and a junior bridesmaid in my daughter's wedding. The advise on measuring the person who will be wearing the garment are right. My grandaughter who was 9 at the time was actually smaller in the waist than her 5 year old sister and cousin. My former boss who did the custom dressmaking always started with the bust measurements when choosing a pattern for a customer. That is the hardest area to fit.
As a really full figured girl all of my life I loved working with knits for myself. They are so forgiving and make fitting a lot easier.