Okay, here's the deal.
First, you shouldn not expect miracles in a week. A healthy rate of losing weight is around 1-2 kilos a month. So be patient.
Regarding the walking...stick to it right now. Your joints are probably not used to over exertion, so don't stress them yet.. IT COULD LEAD TO SERIOUS DAMAGE. Stick to walks for now.
Now diet is the most important. As many have said, cut out the junk. First, calculate your BMR using this:
BMR Calculator
Calculate your BMR and then your daily calorie requirements using the Harris Benedict Equation.
To have a healthy rate of losing weaight, cut your calories by 400-500 of your daily needs. So if your daily requirements are 2,400 cals, consume only 1,900.
Remember, DO NOT FOLLOW ANY FAD DIETS. They're junk. You'll lose only water weight and it will all come back as soon as you drop it.
Follow a balanced diet. Don't skip any meal. Always eat breakfast. Eggs (Boiled) , Chicken, oats, fruits, vegetable. Limit carbs. Take protein. (Chiken Tikka, Boiled chiken). Fish is good too. (Tuna/Sardine cans)
Almost forgot... while dieting, DO NOT consume any calories through liquids. Apart from the obvious (no cold drink), you should also avoid juices, milk etc. These are empty calories... they don't fill you up.
Hope I was of help.