http://www.brianmac.co.uk/hrm1.htm - This website calculates heart rate training zones depending on how difficult you want to make your workout. Figure out your Resting Heart Rate by taking your pulse first thing in the morning when you wake up (lying in bed). With this, you can then calculate your maximum heart rate, as well as upper and lower heart rate training zones.
Heart Rate - This article describes a slightly different way of targeting your heart rate zones.
Perceived Exertion - In addition to monitoring our heart rate, knowing how we should be feeling during exercise is an important concept for when we're exerting ourselves.