Well, there are several things that could be happening. I think that the most likely possibility is that you are accidentally doing a yarnover. There are times when you want to do a yarnover on purpose--for effect, but in your case you don't want this to happen. Be careful not to accidentally bring the yarn over the needle, thus creating an extra stitch. Also, be careful with the first stitch. It's easy to knit into both sides of the loop of the first stitch when your first learning to knit. I have made that mistake, too, a few times!