The pattern and instructions should designate (ie A, B, C...) for the robe or nightshirt. Be sure to use the pattern pieces that signify the nightshirt and ignore the pieces for the robe (be careful though, some parts of the pattern may be used for both). Then be sure to follow the instructions that apply specifically to the nightshirt. Somewhere in the pattern paperwork, perhaps the back of the envelope, there is a breakdown of what pieces you need for which pattern. Also, make sure to lay the pattern pieces out on the fabric as shown, otherwise you will get fabric cut on a crossgrain which will make your garment hang funny.
You may want to check out some books from the library for beginning sewing projects or look for sewing tutorials on the internet. How to cut fabric with paper pattern
According to this, the knee-length nightshirt is labeled as "A." The very beginning of the pattern instructions should show pictures of how to lay out the pattern pieces on the fabric.