You need to figure out how you're adding stitches.... There's a thread
with some ideas, but the most common ways are when you start a row, the yarn is over the top of the needle to the back and it pulls the first stitch up so it looks like two sts. So hold the yarn out to the side a bit until you stick the needle into the first st to knit it. If you're doing a row with knits and purls you might be bringing the yarn over the needle to switch instead of between the needles. Sometimes you might be putting an extra wrap between sts, so watch where your yarn is and make sure it's not around the needle before you begin to make the next stitch.