You're probably quite familiar with your ticker. It pumps blood throughout your body, it flutters when you fall in love...it obviously keeps you alive. Some, unfortunately, are better than others so if you have issues with yours I urge you to consult with a physician before taking any of the advice below to (pardon the pun) heart.
That said, let's talk about target heart rate. Your THR is a very important number because it defines the intensity at which you should be working out. And reaching that intensity is important because it means you're sucking out maximum heart, lung and muscle benefits from your workout. To figure out your THR, you'll need to know your resting and maximum heart rates.
What is your resting heart rate? You definitely don't need a medical degree to figure it out. Simply take your fingers and rest them on a pulse point (wrist, throat) and count the number of beats in 15 seconds then multiply it by four. It's that easy, but there is an optimal time to count. RHR can fluctuate due to sickness, dehydration or over training. It may even lower as your fitness level gets higher. So it never hurts to average a few readings.
On the other end of things, your maximum heart rate is the hardest your heart can pump. A variety of professional tests can be conducted to get an absolute MHR, but you can estimate it with a simple formula:
The more familiar you get with your THR range, the more you can customize your workouts. And the more you can monitor your intensity. (You've seen the charts on the cardio machines, right?)
Don't like the math? Find an easy calculator online. Don't feel like stopping mid-workout to take your heart rate? Find a heart rate monitor at the store.
And, as always, listen to your body. Your heart will be the first to tell you if it needs a break, and I advise you never to let it get to that point.