OP, just to add to GillesHK's excellent advice, the 220 minus your age for max heart rate is a pretty generic way of measuring things. It dosen't take into account individual levels of fitness, genetics etc etc. I'm in my mid 30s and I regularly work past 195. A better indication of cardiovascular fitness is your recovery rate and VO2 Max.
Best thing to use your heart rate monitor is to understand your body by monitoring your heart rate while you exercise, e.g how long can you sustain a certain level of performance so you can achieve your exercise goals. Try to find out your aerobic, anaerobic and red line zones (dont push too hard to get to red line if you are new to exercise initially).
If you are aiming for weight reduction, you will want to burn fat at an aerobic level which involves low intensity training for a much longer period. This is at a zone where you are slightly puffed but not in discomfort with your breathing. The heart rate monitor will help you know when you are working at this zone. There are lots of online calculators like the following link
http://www.brianmac.co.uk/hrm1.htm
which allows you to get an idea based on your age, weight, height etc, and some polar models calculate this for you as well.
But ultimately listen to your body, if you are finding a level of intensity unsustainable, then most probably you have moved out of aerobic to anaerobic or more. Best of luck with your training and enjoy the fruits of a healthy body !