Patterns today are printed with all sizes on one pattern. Read the size chart carefully to determine the best size for you. Use a tape measure to measure your body for the key measurements: Bust, Waist and Hip. The Hip is measured 9" below the waistline.
It is a good idea to highlight the cutting line for your size so you don't confuse the lines while cutting out. Use a contrasting marker to hightlight the correct cutting line.