Ignore the size chart on the back. Measure your bust, waist and hips. The finished measurements will be printed on the tissue. I choose the size that is two inches bigger than my measurements, which makes a fitted dress. You can also measure a dress you already own and compare it to the measurements. This size will be larger than the size you would buy in a shop. You'll need to make a muslin (practice dress) first to work out all your fitting adjustments. I suggest getting a flat sheet from the charity shop to cut up for this.
As to choosing a pattern, never pay full price. Joann's will eventually have them for cheap, check your local sales flyer: http://joann.shoplocal.com/joannsalesflyer/Default.aspx?action=entry&pretailerid=-99845&siteid=228&storeID=2412188
I suggest a Very Easy Vogue pattern: http://voguepatterns.mccall.com/very-easy-vogue-pages-856.php