Is this a good diet, or how can I improve it?

I start out the morning with a cup of coffee with creamer, and eat something like a pop-tart, or something small like that. Then at lunch at school, I eat something that is like a lean cuisine type thing. It's Stromboli and it's about 400 calories. Then for dinner I have something like steak, with a salad (With Ranch) and something like green beans. I also drink 5-8 glasses of water each day, and one glass of juice and/or milk. Any tips on how to improve this, and if possible, tell me how much I would lose in about three weeks.
Thanks.