Remember, always get a medical clearance before starting an exercise program!
When you start out exercising to lose weight, forget intensities, heart rates, and training zones. Just get out there and move. If you start by pushing yourself to the limit every day you may not last very long so take a long-term look at your goals. Start by just getting your body used to exercising again. Do this 4-6 times per week but make sure that it does not make you too sore (this will stop the frequency of your exercise). As your body adjusts you can start increasing the intensity.

So how often should I be training?
Start with every second day and build up to most days. If you have not exercised in along time you may want to start with a 10-15min walk. Try and build this up to 40-60min, then start putting in bursts of short (100m) jogs. If you have trouble jogging, walk up hills or stairs to increase the heart rate.