The holes could be yarn-overs.
When changing from purl to knit, and you move the yarn from front to back to do a knit stitch, make sure the yarn goes between the needles instead of over them.
Same when going from knit to purl, you bring the yarn to the front, and make sure your yarn goes between the needles.
Or, you could just be holding the yarn too loose when you make the switch.
It will get better with practice.