Best way would be to slowly start cutting out and adding different foods, along with adding some excecise. vegetables and fruits are great to replace junk food with (even if you add chocolate and what not on some fruit here and there :) ) Just dont change everything at once cause that could throw your body out of wack. Hope this helps :)
In my opinion, you should start by setting goals that are practical and bear in mind that to be successful you must be disciplined and consistent. In addition it is important to eat a healthy balanced diet as exercise alone does not work.
These two other answers are good: also keep in mind to eat only low gi carbs (glycemic index) like whole grains such as wheat bread and brown rice, oatmeal, granola, sweet potatoes. And try to cut carbs out no later then 1 or 2 pm. Eat more green veggies ( fresh and steamed) and lean white meats along with drinking more water daily. This will help! And if you can, exercise!