When I first started crocheting, I didn't know a whole lot about yarn. I went into the store and grabbed a skein based solely on it's color. If I liked the color, I picked it up. For my first couple of projects (blanket and scarves), this system didn't matter much. (I guess I got lucky there.) Then I decided I wanted to make a sweater from a pattern book. I went into the store looking for a color. I was about half way through the pattern before I realized that it was going to be way too large for me (or most people I knew) to actually wear. I ended up scrapping the project.
I worked out the pattern successfully. My mistake? I didn't pay attention to the yarn. My pattern called for a super fine (weight of 1) yarn and I was using a light (3) yarn. This made my "medium" sweater turn into an extra, extra, extra large.
For many beginners, reading yarn labels can be a tricky thing. Here's a link that has a brief breakdown on how to read yarn labels and what all those little symbols mean.