I need to lose weight!?

I have put on three stone in the past two years! Two of those stones have come on in the past half a year. I used to have a stunning size 8 figure and now I'm a sloppy 14. I have no idea how this has happened! I need to lose at least half a stone in the next month or so, I know exercise helps, I have starting jogging, I have also starting biking to school and work. I need to lose it quickly, has anyone got any advise how to lose it quickly, diet plans, exercise tips ect?