Hi There Mir, First of all, good luck to you. Knowing you need to make a change is the first step. You will need to decide on a plan to follow, whether it is a High Protein/Low Carb option like Atkins, or a or a plan like the Alkaline diet, which is primarily leafy green based. You should pick your plan, buy a book on it, and educate yourself.
Whatever plan you decide on, you will be cutting out the junk food, desserts, and processed foods. Don't believe what some people will tell you regarding "Cheat" days. I can tell you from experience that one cheat day will destroy a whole week of proper eating. Pick your plan and stick to it. Period.
Finally, for exercise, you will need to do more than simply walking. You need to stimulate your body to build some muscle. Lifting with free weights, and interval training will give you much more bang for the buck.
Take care, and good luck...Kevin