Heart rate monitors are a great way of measuring your effort during training and racing and are now thankfully much cheaper than they once were. It doesn't take much to improve your training and once you start measuring and judging your effort from day to day, you should start to see the benefits.
To train effectively with a HRM you need to work out your training zones. There are a few ways of doing this, but a word of warning - the old method of taking the number 220 and then minusing your age, is not the most accurate.
Using a ramp test you can work out your maximum heart rate. But be warned, they are brutal!
Your functional threshold heart rate is probably the most useful number to work out. From there you can work out your training zones accurately and any heart rate above FTHR is anaerobic and unsustainable for a long duration.