First change your diet.
Then exercise, increasing the amount gradually and making sure you variate.
I was a big'un, then I changed.
I went from kebab and chips and playstation every night to cereals, fruit and 3 mile runs every night.
It takes time though mate, you can make a dent in 3 months but keeping it consistent takes alot of time and effort. Start by making little changes and increase them every week. Don't crash change becuase youll do more harm than good.