A good guideline is about half an hour a day of moderate exercise (a very light jog depending on your level of fitness).
A little less if you exercise more vigourously, a little more if you exercise less vigourously.
It's quite important to do some strength building exercises to increase muscle mass.
Push ups, squats and abdominal crunches are the simplest and require no special equipment.
Start with just a few repetitons (3 a day) and build up gradually (by say one repetition a week).
Gentle stretches before and after exercise are always helpful too.
I hope this helps.