While Biggan makes patterns so that straight or circular knitting needles may be used, she prefers to use circular knitting needles because they not only allow for fluid motifs, but they also reduce stress and strain on your shoulders and limit the number of seams in the garment you are knitting.
The type of knitting needle you need will depend on the pattern you are using. Every pattern will specify which types of knitting needles should be used, as well as the size of the knitting needle, for optimal productivity and appearance.
If you’re a beginner, however, it may be easiest to use straight knitting needles while you practice and hone the knit and purl stitches. Then, once you are comfortable with these two basic stitches, try switching to the knitting needles specified in the pattern you’re using.