When crocheting, how do I make all my rows the same length?

I took up crocheting, and everything I do ends up as a triangle. for example, I start a chain with 15 sc, then I sc the second row, still 15 chains, and somewhere on the 4-5th row there are only 14 chains, and so on until i end up with 12 chains on my row. Is there something i'm missing? Is there a really common error im doing? this a href="http://crochet.about.com/library/blbegscarf.htm" rel="nofollow"http://crochet.about.com/library/blbegsc…/a is what im trying to do.