I second Jan in CA and Suzeeq.......when I first started knitting, I also began with a scarf, and I noticed that it kept getting wider and wider. I finally figured out that I was doing the two things that Jan in CA said - I was accidentally making a yarn over, and I was also knitting into the wrong part of the stitch. What I would suggest doing to confirm that you're accidentally making increases is to count your stitches as you knit, and then go back and count your stitches after you're done with a row. If you had cast on 15 stitches originally, and find that you have 16 or 17 stitches after your first row, then you know you're accidentally increasing. If this is the case, I can give you a youtube link that shows you exactly what you're doing wrong.
Happy Knitting! :knitting: