I agree with desperate. Check your how many calories you should be consuming and don't go too much above or below that. The key to diet success is always a balance between regular (moderate to intense) exercise with cardio and weight components, as well as a calorie deficiency. The fastest and easiest way to lose weight and keep it off is making exercise a priority and committing to a healthy, nutritious diet. I was about 265 at my heaviest and I am down to 206 as of today and still going. I have been at it for about 4 1/2 months now. Give yourself a realistic goal. Maybe make small goals such as 10 or 15 pounds at a time. Once you start with a good exercise/diet regimen you will most likely see fairly immediate results. You may hit what is called a "plateau" where you stop losing even doing your normal work outs and diet but don't let that discourage you. It is usually temporary and you may not even experience that at all. Just go for it! If you don't already belong to a gym- invest in a membership!! I think by December you could definitely be pretty close to that goal, just keep in mind that if you lose too fast it might be harder to keep it off.
Good luck, and I'm sure you will achieve your goal!