You might find some help here: http://www.knittinghelp.com/videos/knitting-tips. Just watching someone else work can help me figure out what I'm doing wrong.
What stitch(es) are you using? Are you knitting every row for garter stitch, or are you alternating knit and purl rows for stockinette? Oftentimes extra stitches appear because the yarn goes over the needle between two stitches or at the end of the row instead of under the needle. If you're changing between knit and purl within a row, you might be moving the yarn from front to back or back to front so that you create a yarn over. The yarn lying over the needle gets knitted or purled as if it were a normal stitch when you work the next row.
It sounds like you're really making good progress in your knitting adventure. Stick with it and you'll get the hang of it.