The best day to embark on any change in lifestyle is the day you fully believe in yourself to achieve it.If you are dieting for weight loss it's quite simple - eat less than you burn, or burn more than you eat, or both. But for optimum nutrition it's recommended to eat a minimum of 1200 calories per day, with an increase in 100-200 increments after reaching your goal weight - if you are exercising (cardio is good for fat loss, pilates or yoga is good for muscle strength and tone) you should be able to maintain your new weight, provided it is natural for your body to be there; if you aim lower in weight than your body can maintain at 1800, then it is not a natural weight for you, and cannot be maintained without starvation which comes with a whole loads of medical issues and deficiencies. There are many source of calorie amounts in food on the net, Caloriecounter is a good source.
A diet comprised of predominantly vegetables, fish, nuts, seeds, and a couple of servings of fruit per day, is likely to be the best nutrionally. But this is dependant on your physical health. If you have any medical conditions it would be best to consult a nutritionist prior to starting any new nutritional lifestylr.
If you want the effects of your changes to be longterm, you must treat them as 'lifestyle changes', new habits to replace old habits that don't work for your wellbeing.