If you're new to doing cardio such as biking or running, and you want to know if you are working at the right intensity for your body, using a heart rate monitor is a great idea. Whether you are doing cardio to lose weight, for fun, or to build your endurance for competitions, a heart rate monitor will give you an accurate readout of how hard your heart is working.
Shopping for a heart rate monitor can be a bit overwhelming since there are so many different kinds. Plus they can be pretty pricey depending on how many bells and whistles they come with. If you are just starting out, there's no reason to spend tons of money if all you want to track is your heart rate.
The Polar FS1 heart rate monitor is a great, basic monitor. The black transmitter band goes against your skin and around your chest (you can stick it under the elastic of your sports bra). The watch on your wrist watch continuously tracks your heart rate so you'll know when you reach your target heart zone. The watch even gives you visual and audible alarms to keep you in your target zone. The watch also keeps track of how long you've been exercising, so you can figure out how long you've been "in the zone" during your workout.
You can order one from REI.com for only $59.95, which is an awesome deal on a basic heart rate monitor. When you use one, you can actually track your progress and see improvement in your endurance, so I found that it was just the thing to motivate me to workout.
Not sure what your target heart rate is? Check out the FitSugar calculator it will help you find your zone.