First of all, anyone overweight or obese should contact their doctor before starting a new workout plan.  One of the biggest concerns for anyone who is overweight and wanting to start an exercise routine is the heart.  Exercise is strenuous on the body and when someone is obese they tend to have a weaker heart or other heart related conditions.  Breathing may become more difficult during exercise making it harder to work out as long as needed for any result.

Osteoarthritis of weight bearing joints can also be a problem for some people because this causes pain, stiffness, and locking of the joints due to the amount of weight they bear.  This can make it both painful and difficult to do many exercises.  It is usually best to do cardio exercises for a few weeks to start losing the weight before starting a more demanding exercise like muscle workouts.  The key is to start slow with only lifting as much weight as what is comfortable.  The best muscle workouts you can probably perform are those using your own weight as resistance such as sit-ups and squats.  Believe it or not, the more muscle you are able to put on, the more it will actually help you to lose the weight.