900-1200 calories a day (or less if you find you simply aren't hungry for it; the less, the faster), lots of water, any food at all. Drink water when you think you're hungry, it will keep you hydrated, reduce your appetite, and help you determine how hungry you really are.
On this "plan" I lost twelve pounds in a week and a half, through Thanksgiving weekend no less.
You're also safe fasting for a day or two at a time.
And, even though there is an extremely small chance of developing a vitamin deficiency depending on what you exclude, consider taking a daily vitamin.
Balanced dieting is important, but only for the long-term. The huge majority of people can go on a "bizarre", unbalanced diet to lose weight for weeks or months at a time with no ill effect.
But do be careful, you're already close to being underweight.
