Count how many stitches you have on the needle and see if it is the same number of stitches that you cast on. If it's more than you cast on, you may be making a mistake at the beginning of each row by pulling up on the first stitch so that it looks like two stitches. If you have fewer stitches than cast on, you may be dropping stitches or knitting two stitches together. If you have the same number of stitches, your tension may be getting tighter or looser as you knit. In that casse, just keep knitting and eventually the tension will even out.
This is all good because you're making a beginning and learning as you go.