Several basics. Find an activity that keeps them outside & active and share it with them if it takes it. Remove the tv or computer from their room if there is one. Limit tv+non school computer time to 2hrs/day & have tv off during mealtime. Don't allow eating during tv viewing or computer work. Serve nutritious meals & don't let them skip any. Assure kid has ample time for sleep, preferably 8-9hr/d.
No junk food. Everything dr. Ferguson stated and I would add, do it as a family, everyone eats and snacks the same thing, donate the junk food and the sugary drinks, shop with a list and do not buy anything that is not on it. Make it fun! Do not use the "d" word (as in diet) provide your child with life long tools to be healthy! Let him help with food prep and maybe grow a little garden together.