At this time of year it is common for people to make unrealistic diet and fitness resolutions. There is no such thing as overnight success with a diet or fitness regime. It takes time and commitment.
The best way to go about it is to break your goal into achievable sized goals. Make a list of diet and lifestyle habits that you know you will need to enforce in order to reach your goal. Some habits you will find easy to implement and these should be applied straight away. The habits you find more challenging to put into action should be tackled one at a time if you are to succeed in achieving long-term success.
Slow progress is better than no progress!