If you started with 12 sts and they now fill up your needle, you've added them and you shouldn't unless you're doing it deliberately. It would help to know what you're making.
Watch at the beginning of the row, if the yarn is up over the top of the needle to the back it pulls up the first st and each leg looks like a separate stitch and if you knit into both of them you get more sts. Hold the yarn out a little to the side so you can see the one leg you're supposed to knit into. If you're mixing knits and purls on the same row you move the yarn between the needle tips, not over the needle. The yarn over the needle is called a 'yarnover' or YO and can be used to make new sts or put holes in the work (lace) but should have a decrease for every one so the stitch number stays the same.
If you're deliberately increasing stitches and want to do so, then you need to get a circular needle to hold all the stitches.