I recommend getting a good heart rate monitor, to make certain that you stay within the 60% - 70% heart rate range during cardio exercise. Once you reach this point, realize that you burn the most body fat between 30-40 minutes of exercise, and to a maximum of 60 minutes of consistent cardio exercise. Once you reach approximately 60 minutes, your body will start to burn muscle instead of fat.
You can perform cardio exercise every day if you like, and staying within this 'comfortable, yet productive' range will assure that you are not burning yourself out. If you get bored with your exercise routine, you can always change it up. For example, when you get bored of using the treadmill, try an elliptical exercise machine, recumbant or stationary bicycle.
Past that, what do you call a 'healthy diet', and how much body fat do you want to lose? You should look at what you have now, and accurately track your body fat percentage. Don't go by the BMI, if you can help it, because it's not great for all people.
