Are you doing just a knit stitch, or knits and purls? Most likely, you are either knitting with the yarn in front, or purling with the yarn in back, either of which loops the yarn over the needle in between stitches and causes you to have extra stitches. When you do this on purpose it's called a yarn over.
It's a common mistake for new knitters.
Mommy to two wonderful girls
"I don't have to conform to the vagaries of time and space, I'm a loony!" -Campbell Bain (David Tennant) 'Takin' Over the Asylum'
OTN: Cabled Sweater
, arm warmers, kids' clothes