Why am I not losing weight?

I have been dieting and working out for 19 days now. I want to lose 10 pounds this summer! I am taking this really seriously so I've cut refined sugar out of my diet. I haven't had it in 19 days. Along with that I have been basically only eating veggies, fruits, chicken, eggs, turkey, almonds, and wheat bread. I only drink water. I have gotten back to when I don't eat after 7 at night. I try to workout everyday. I have a gym membership and I have been getting on the treadmill almost an hour everyday along with arm exercises. I try to burn at least 400 calories a day or even more. Today I burned 500 calories. BUT I haven't lost one pound. I weighed myself yesterday and I weighed the exact amount I did before I started. How can this be?? I am 5'6 and currently weigh 142 pounds...arg! I just want to go back to school feeling more confident and slimmer. :(