The best way of losing weight is by lifestyle changes. The key is that, if your output (the amount of calories you burn in your daily activities and through exercise) is more than your input (the amount of calories that you eat), then you are bound to lose weight. On the other hand, if your input is more than your output, you are bound to gain weight. So, these are the two things you have to monitor: you have to increase your output and at the same time, decrease your input.

Decreasing calorie intake is the key to losing weight. Increasing physical exercise is the key to keeping it from coming back. So you have to do both to keep it down.

The very fact that you find it difficult now to walk shows that you must do it. You should consult a physiotherapist and begin a controlled exercise regime under him. Go ahead and try, you will find your whole world will change.

Here are some excellent govt. sites that you can go through for some guidance:
http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/patmats.htm (Follow all the links given here, you will be able to find the program of your choice).

http://www.patient.co.uk/showdoc/23068959/ (general useful tips)
http://www.patient.co.uk/showdoc/23068858/ (general useful tips)
http://win.niddk.nih.gov/publications/index.htm (Further Info)