Look at your yarn label...it will have a little box that tells you what size your needle "should" be.
Look at the end of your needle. Is that size bigger than the size on the yarn? If it is, you are knitting at a looser gauge.
If that isn't the problem, I would strongly suggest checking out some of the videos on this site (great close up videos of Amy's hands as she knits and explains what she is doing!) It could be that you are creating yarn overs accidentally. Look at the outer edges of you work? Is it getting bigger? A yarn over is a way to increase the number of stitches in a row.