I am a very overweight man at 400lbs (181 kg). I've been told by several doctors that I need to lose weight or else my health will get worse and worse.

What exercises are safe for a large person? For weight training I'm wary of doing squats since every time I've tried I end up with sore knees and have problems walking. I've also found that most cardio programs have the same effect.

For losing weight should I be concentrating on cardio or weight training? Is it better to exercise or should I just change my eating habits?

What are some good websites for teaching myself proper nutritional information? I've heard the old adage "eat less", but considering that most of my meals are from fast food restaurants, I have no idea of what kinds or proportions of foods I should be eating.