I also recommend a Polar HR monitor. There are several models to choose ranging from a relatively inexpensive model that only displays HR to more expensive models which have more bells and whistles. I think the least expensive model is more than adequate.
There are a couple of reasons to use a HR monitor: 1) Is makes your workout more entertaining (for someone who would rather lift weights than do my cardio, this is a big plus

), and 2) using your heart rate as a tool to monitor/modify your workout will more effectively and efficiently improve your cardiovascular fitness level.
There are few good books on heart zone training that are worth reading:
The Heart Rate Guidebook to Heart Zone Training by Sally Edwards
Total Heart Rate Training: Customize and Maximize Your Workout Using a Heart Rate Monitor by Joe Friel
Sally's book is a great one from beginners. Joe's book is a bit more advanced.